feat: PHP language support

[makinzm]

Summary

• Add PHP file analysis support using tree-sitter-php 0.22.8
• Parse use statements: simple, aliased (as), grouped ({}), use function, use const
• Classify imports as Internal / External / Stdlib (PHP built-ins like DateTime, PDO, Exception)
• Auto-detect base namespace from composer.json autoload.psr-4
• [resolve.php] config section with namespace and composer_json keys
• .php extension registered in ext_to_language so mille init detects PHP projects

Changes

• src/infrastructure/parser/php.rs — PhpParser with tree-sitter AST walking
• src/infrastructure/resolver/php.rs — PhpResolver with Internal/External/Stdlib classification + read_namespace_from_composer_content
• src/infrastructure/parser/mod.rs — dispatch .php to PhpParser
• src/infrastructure/resolver/mod.rs — dispatch .php to PhpResolver, read [resolve.php] config
• src/usecase/init.rs — "php" => Some("php") in ext_to_language
• Cargo.toml — tree-sitter-php = "0.22"
• README.md — PHP added to language support table, [resolve.php] reference, Laravel example config
• docs/TODO.md — PHP support noted in summary and completed task list

Test plan

• RED phase committed with --no-verify (10 parser tests failing with todo!())
• GREEN phase: all 10 parser tests + 12 resolver tests pass
• Full test suite: 348 tests pass, no regressions
• cargo fmt applied, cargo clippy clean
• lefthook pre-commit hook passes

Parser test coverage

• test_parse_php_simple_use — use App\Models\User;
• test_parse_php_aliased_use — alias ignored, original path returned
• test_parse_php_group_use — use App\Services\{Auth, Logger}; expands to 2 imports
• test_parse_php_function_use — use function App\Helpers\format_date;
• test_parse_php_const_use — use const App\Config\MAX_RETRIES;
• test_parse_php_multiple_use — 3 mixed use statements
• test_parse_php_no_imports — empty for plain class file
• test_parse_php_names_class, _function, _comment

Resolver test coverage

• Internal classification (base namespace match)
• Stdlib classification: DateTime, PDO, Exception, \DateTime (leading backslash stripped)
• External classification (third-party vendor namespaces)
• test_php_resolver_internal_resolved_path — App\Models\User → App/Models/User.php
• test_php_resolver_external_no_path — no resolved_path for External
• test_read_namespace_from_composer — PSR-4 key extraction
• Empty base namespace → all External (except Stdlib)

Notes

• use require/include are intentionally not parsed (as per scope)
• mille init PHP support (scanning imports to generate layers) is not yet implemented — would require a follow-up PR

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