chore: Simplify extension

[attitude]

Summary

• providers.ts: Extracted delegateLocationProvider() helper shared by PHPXDefinitionProvider, PHPXTypeDefinitionProvider, and PHPXImplementationProvider — eliminated ~60 lines of triplicated boilerplate. Simplified mapLocationLinkToSource using a single remapUri variable instead of three mutable lets. Removed dead PHPXWorkspaceSymbolProvider class (exported but never registered in extension.ts).
• compiler.ts: Moved stderrOverflow declaration above its handler (was declared one line after use). Added stderrOverflow to the close handler's overflow branch (previously only truncated was checked, leaving stderr-overflow kills to fall into the code !== 0 path and attempt JSON.parse on a truncated buffer). Removed dead compilationErrors map and getCompilationError() method (the map was written but never read externally). Fixed err.message || err.stack order in compileAndWrite catch — stack is now used for logging, message for the user-facing error.
• diagnostics.ts: Removed shouldForwardDiagnostic() stub (always returned true) and its no-op .filter() call site.
• positionMapper.ts: Removed /g flag from module-level PHP_WORD_PATTERN constant — the flag implies mutable lastIndex state, but all use sites correctly create fresh instances via new RegExp(PHP_WORD_PATTERN.source, 'g'); the constant is used only as a source holder. Fixed double-statSync on cache miss: now does statSync first, then readFileSync (one syscall saved per cold-cache file access).
• languageClient.ts: Removed redundant ?? undefined from optional-chain expression.
• extension.ts: Deduplicated the three identical vendorWatcher event handlers into a shared clearCompilerCache callback.

Test plan

• TypeScript compilation passes (npm run compile — no errors)
• Go-to-definition, type definition, and find implementation still work in .phpx files
• stderr overflow during compilation now produces "Compilation output exceeded size limit" instead of a JSON parse error
• No regression in diagnostics forwarding from PHP language server to .phpx files

🤖 Generated with Claude Code

[attitude]

Superseded by #25