Add LSP diagnostic test files for multiple languages

Includes test files for Lua, JavaScript, PHP to verify the Paper Tonic
Modern colorscheme's diagnostic virtual text colors across different
LSPs and their expected diagnostics.

TEST_DIAGNOSTICS.md

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+# LSP Diagnostic Test Files
+
+These files are designed to trigger LSP diagnostics of all types to test the Paper Tonic Modern colorscheme's diagnostic virtual text colors.
+
+## Test Files
+
+### test-diagnostics.lua
+Lua file with intentional errors for lua_ls (Lua Language Server).
+
+**Expected diagnostics:**
+- **Errors** (red): Undefined variables/functions, type mismatches, invalid operations, wrong argument counts
+- **Warnings** (orange): Unused variables, shadowing, unreachable code, lowercase globals
+- **Hints** (green): Redundant code, style issues
+
+**Note**: lua_ls provides the best mix of diagnostic levels.
+
+### test-diagnostics.php
+PHP file with intentional errors for Intelephense (PHP Language Server).
+
+**Expected diagnostics:**
+- **Errors** (red): Most issues appear as errors by default in Intelephense
+  - Undefined functions/variables
+  - Type mismatches
+  - Wrong argument counts
+  - Invalid method calls
+
+**Note**: Intelephense is strict and reports most issues as errors. To see warnings/hints, configure in `.nvim.lua`:
+```lua
+return {
+  lsp = {
+    intelephense = {
+      settings = {
+        intelephense = {
+          diagnostics = {
+            undefinedSymbols = true,
+            undefinedVariables = true,
+            unusedSymbols = "hint",  -- Show unused as hints
+          }
+        }
+      }
+    }
+  }
+}
+```
+
+### test-diagnostics.js
+JavaScript file with intentional errors for ts_ls (TypeScript Language Server).
+
+**Expected diagnostics:**
+- **Errors** (red): Undefined variables/functions, invalid property access, missing parameters
+- **Warnings** (orange): Unreachable code, type comparison issues, duplicate cases
+- **Hints** (green): const vs let suggestions
+- **Info** (blue): Unnecessary operations
+
+## How to Use
+
+1. Open any test file in Neovim:
+   ```bash
+   nvim test-diagnostics.lua
+   # or
+   nvim test-diagnostics.php
+   # or
+   nvim test-diagnostics.js
+   ```
+
+2. Wait for LSP to attach and analyze the file (1-2 seconds)
+
+3. Check the virtual text diagnostics at the end of lines with issues
+
+4. Use `:LspInfo` to verify the LSP server is attached
+
+## Color Reference
+
+The Paper Tonic Modern colorscheme uses these colors for diagnostics:
+
+- **DiagnosticError** (virtual text): `#d70000` (bright red) on `#ffefef` background
+- **DiagnosticWarn** (virtual text): `#d75f00` (orange) on `#ece0e0` background
+- **DiagnosticInfo** (virtual text): `#8989af` (blue) on `#e0e0ec` background
+- **DiagnosticHint** (virtual text): `#89af89` (green) on `#e0ece0` background
+
+## Testing Checklist
+
+- [ ] Errors show in red with red undercurl
+- [ ] Warnings show in orange with orange undercurl
+- [ ] Hints show in green
+- [ ] Info messages show in blue
+- [ ] Virtual text is readable against light background
+- [ ] Sign column icons show correct colors
+- [ ] Diagnostic floating windows display correctly
+
+## Debug Command
+
+Use the `<leader>hi` keymap to inspect highlight info under cursor and verify diagnostic colors are applied correctly.

test-diagnostics.js

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+/**
+ * Test file for JavaScript/TypeScript LSP diagnostics
+ * Tests error, warning, info, and hint virtual text colors
+ */
+
+// ERROR: Undefined variable
+console.log(undefinedVariable);
+
+// ERROR: Undefined function
+nonexistentFunction();
+
+// WARNING: Unused variable
+const unusedVariable = "I'm never used";
+
+// ERROR: Type error (if using TypeScript or JSDoc)
+/**
+ * @param {string} name
+ * @param {number} age
+ */
+function greet(name, age) {
+  console.log(`Hello ${name}, you are ${age}`);
+}
+greet("Alice", "not a number");  // Type mismatch
+
+// WARNING: Unreachable code
+function earlyReturn() {
+  return "done";
+  console.log("Never executed");  // Unreachable
+}
+
+// ERROR: Invalid property access
+const obj = {};
+console.log(obj.nonexistent.nested.property);
+
+// WARNING: Comparison with different types
+if ("5" == 5) {  // Should use ===
+  console.log("loose equality");
+}
+
+// ERROR: Missing parameter
+greet("Bob");  // Missing age parameter
+
+// INFO: Unnecessary type assertion (TypeScript)
+const num = 123;
+const str = String(num);  // Could be simplified
+
+// HINT: Variable can be const instead of let
+let neverReassigned = "should be const";
+console.log(neverReassigned);
+
+// ERROR: Division by zero (some linters catch this)
+const result = 10 / 0;
+
+// WARNING: Duplicate case label
+const value = 1;
+switch (value) {
+  case 1:
+    console.log("one");
+    break;
+  case 1:  // Duplicate case
+    console.log("one again");
+    break;
+}
+
+// ERROR: Invalid this context
+const obj2 = {
+  name: "test",
+  greet: () => {
+    console.log(this.name);  // Arrow function doesn't bind this
+  }
+};
+
+console.log("JavaScript diagnostic test complete!");

test-diagnostics.lua

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+-- Test file for LSP diagnostics - lua_ls
+-- This file intentionally contains different diagnostic levels
+-- to test diagnostic virtual text colors in paper-tonic-modern
+
+-- Define some valid code first
+local valid_function = function()
+  return "valid"
+end
+
+---@diagnostic disable-next-line: unused-local
+local _ = valid_function()
+
+-- ERROR: Undefined global (lua_ls marks this as error)
+print(undefined_global_variable)
+
+-- ERROR: Undefined function
+undefined_function()
+
+-- WARNING: Unused local variable (lua_ls warning level)
+local unused_variable = "I'm never used"
+
+-- WARNING: Lowercase global (should be local)
+lowercase_global = "This should be local or uppercase"
+
+-- HINT: Redundant parentheses (some configs treat as hint)
+local x = (5)
+print(x)
+
+-- ERROR: Wrong number of arguments
+local function takes_two_args(a, b)
+  return a + b
+end
+takes_two_args(1)  -- Missing second argument (lua_ls error)
+
+-- ERROR: Attempting to call a non-function
+local not_a_function = "string"
+not_a_function()
+
+-- WARNING: Shadowing outer local
+local shadow_me = 1
+do
+  local shadow_me = 2  -- Shadows outer local (warning)
+  print(shadow_me)
+end
+
+-- ERROR: Invalid table access
+local empty_table = {}
+print(empty_table.nonexistent.deeply.nested.property)
+
+-- WARNING/HINT: Can be simplified
+if true == true then  -- Redundant comparison
+  print("always true")
+end
+
+-- ERROR: Type annotation mismatch
+---@type string
+local should_be_string = 123  -- Assigning number to string type
+
+-- WARNING: Unreachable code after return
+local function has_dead_code()
+  return "done"
+  print("This will never execute")  -- Unreachable
+end
+
+-- Valid function call
+print(takes_two_args(1, 2))
+
+-- ERROR: Calling vim API that doesn't exist
+vim.nonexistent_function()
+
+-- HINT: Trailing whitespace or style issues (if configured)
+local with_trailing_space = "value"   
+print(with_trailing_space)
+
+-- ERROR: Parameter type mismatch
+---@param name string
+---@param age number
+local function greet(name, age)
+  print("Hello, " .. name .. ", you are " .. age)
+end
+greet(123, "not a number")  -- Wrong types
+
+print("Diagnostic test complete!")
+

test-diagnostics.php

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+<?php
+/**
+ * Test file for PHP LSP diagnostics
+ * Tests error, warning, info, and hint virtual text colors
+ * 
+ * Note: Configure Intelephense to show different diagnostic levels:
+ * - diagnostics.undefinedSymbols = true (errors)
+ * - diagnostics.undefinedVariables = true (warnings)
+ * - diagnostics.unusedSymbols = "hint" (hints)
+ */
+
+// Define some valid functions and variables first to avoid cascading errors
+function validFunction(): void {
+    echo "This is valid\n";
+}
+
+$definedVariable = "I exist";
+
+// ERROR: Undefined function
+someUndefinedFunction();
+
+// ERROR: Undefined variable
+echo $completelyUndefined;
+
+// WARNING/HINT: Unused variable (depends on Intelephense config)
+$unused = "Never used anywhere";
+
+// Valid code to separate errors
+validFunction();
+
+// ERROR: Wrong number of arguments
+function requiresTwoArgs(string $a, string $b): void {
+    echo "$a $b\n";
+}
+requiresTwoArgs("only one");  // Missing second argument
+
+// ERROR: Type mismatch
+function expectsString(string $param): void {
+    echo $param;
+}
+expectsString(123);  // Passing int to string parameter
+
+// WARNING: Variable might not be defined in all code paths
+if (rand(0, 1)) {
+    $maybeUndefined = "sometimes defined";
+}
+// Commenting out to reduce errors: echo $maybeUndefined;
+
+// Valid usage
+echo $definedVariable . "\n";
+
+// ERROR: Calling method on non-object
+$notAnObject = "string";
+$notAnObject->someMethod();
+
+// ERROR: Invalid array access (multiple levels)
+$emptyArray = [];
+echo $emptyArray['key']['nested']['deep'];
+
+// Valid function call
+requiresTwoArgs("hello", "world");
+
+// HINT/WARNING: Unreachable code (some configs)
+function hasDeadCode(): string {
+    return "done";
+    echo "This is unreachable";  // Dead code
+}
+
+echo "Test complete!\n";
+