Allow file indexes to start at a selected file

[adamziel]

What it does

Allows endpoint_file_index() to accept list_dir pointing at a single file, not only a directory. The response emits a normal file-index entry for that selected file.

Rationale

--only file selections should be able to reuse the existing file-index / skip-defaults flow without indexing the selected file’s parent directory.

Implementation

• Accepts readable files in the list_dir validation path.
• Keeps directories in the traversal stack and handles explicit files separately.
• Adds file_index_item_for_path() so direct-file entries use the same metadata shape as normal traversal.
• Adds unit coverage for a single selected theme file.

Testing instructions

• cd tests && ../vendor/bin/phpunit --colors=never FileIndexSkipDefaultsTest.php
• composer analyze

[github-actions]

Pull pipeline performance — large-directory

Site: large-directory · 2,000+ plus targeted file-transfer scenarios files · 10,000 posts · 25,000 postmeta · PHP 8.5.7

Stage	PR	trunk	Δ	Status	Details
playground-sqlite-db-pull	9.58 s	9.58 s	⚪ +5 ms (+0.1%)	✓	condition=db-pull in PHP.wasm runtime=php.wasm 8.3 wp_mysql_parser=enabled mode=lexer native_lexer=verified native_token_stream=WP_MySQL_Native_Token_Stream native_token_count=18 native_parser=selected trunk: condition=db-pull in PHP.wasm runtime=php.wasm 8.3 wp_mysql_parser=enabled mode=lexer native_lexer=verified native_token_stream=WP_MySQL_Native_Token_Stream native_token_count=18 native_parser=selected
playground-sqlite-db-apply	3.77 s	3.66 s	⚪ +105 ms (+2.9%)	✓	condition=db-apply to SQLite in PHP.wasm runtime=php.wasm 8.3 wp_mysql_parser=enabled mode=parser native_lexer=verified native_token_stream=WP_MySQL_Native_Token_Stream native_token_count=18 native_parser=verified native_ast=WP_MySQL_Native_Parser_Node sqlite_driver_parser=verified trunk: condition=db-apply to SQLite in PHP.wasm runtime=php.wasm 8.3 wp_mysql_parser=enabled mode=parser native_lexer=verified native_token_stream=WP_MySQL_Native_Token_Stream native_token_count=18 native_parser=verified native_ast=WP_MySQL_Native_Parser_Node sqlite_driver_parser=verified
Total	13.35 s	13.24 s	⚪ +110 ms (+0.8%)

_{Numbers carry runner noise; treat single-run deltas as directional, not authoritative.}

_{📈 Trunk performance history — commit-by-commit timeline.}

[adamziel]

Keeping this open: it's a prerequisite for the push stack's scoped reindex (#315, step 6) — 'reindex just the paths that changed locally' needs single-file index entries exactly like this.