[Push] Store file indexes under target relationship directories

[adamziel]

This puts each target URL in its own remote-<hash>/ directory, so files-pull, files-diff, and files-push share one relationship-local index without colliding with another target.

The hash uses the target URL after removing URL user-info, SECRET_KEY, and the site-export-api endpoint alias. It does not include the local tree. A caller which uses another local document root must use another --state-dir.

The pull files now use .remote-index.jsonl, .remote-index.next.jsonl, .local-index.jsonl, pull-plan.jsonl, and pull-index-updates.wal. WAL records carry explicit remote and local fields; replay updates the remote projection first, then the local projection, and clears the WAL only after both replacements complete.

The relationship directory owns the file indexes and push state. Old index filenames are removed rather than kept as aliases.

Stacked on #406.

[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 8.5.9

Stage	PR	trunk	Δ	Status	Details
playground-sqlite-db-pull	7.23 s	7.15 s	⚪ +85 ms (+1.2%)	✓	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	2.86 s	2.87 s	⚪ -5 ms (-0.2%)	✓	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	10.10 s	10.02 s	⚪ +80 ms (+0.8%)

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

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