exact URL origin-length boundary

Parent: #140 Tracked by: #134, #163, #164

Symbols (URL cluster sub-slice 3 — origin length)

Both pinned profiles declare the identical zig-side signature (latin1_slice: [*]const u8, len: usize) u32 (the C++ size_t return narrows to u32).

Last standalone URL-cluster symbol; after this slice only the DOMURL object-creation group (BunString__toURL/toJSDOMURL + WebCore__DOMURL__cast_/href_/pathname_/fileSystemPath) remains in the cluster.

Acceptance criteria

• Parse the latin-1 slice through the allocator-first urlParse with no base; empty/invalid (scheme-less) input returns 0, matching Bun's validity guard.
• pathStart is computed from the parsed parts so it exactly mirrors urlSerialize's prefix emission: scheme: plus, when a host component is present, // + userinfo (user[:pw]@, emitted only when non-empty) + host + (:port when set), plus the /. sentinel for a non-opaque path that begins with //. This is the byte offset where the path begins in the canonical serialization — i.e. url.origin.length for tuple origins, and the scheme/authority prefix length otherwise.
• The latin-1 slice is widened to WTF-8 before parsing (WTF 8-bit string semantics); non-ASCII bytes follow the parser's existing rules. A null slice returns 0 (zig-js sentinel where Bun dereferences unconditionally).
• Compiled runtime fixture covers: http with userinfo+port, default-port elision, file: (empty authority present), opaque-path (mailto:), the //-led non-opaque sentinel case, invalid input → 0, null → 0; results cross-checked against new URL(...).origin.length where the JS getter exists (http/https) and against the serialized prefix elsewhere. Fixture symbol count 313 → 314.
• Home inventory 311/431, Bun inventory 304/421; bun-private-abi-audit + c-api-audit green; full zig build test zero new failures (worktree baseline-vs-candidate against the same pinned base).

Design constraints

• No new parser machinery: reuse urlParse + the parts layout; the prefix-length arithmetic must not duplicate serialization logic beyond reading the same component fields.
• Context-free: no realm, no exception channel — pure bytes in, length out.

Evidence — landed on main as runtime a95eb6fd + docs a44593a7.

Implementation: URL__originLength in src/c_api.zig widens the latin-1 slice to WTF-8 (privateLatin1ToWTF8, WTF 8-bit string semantics), parses with no base through the allocator-first urlParse, and returns the pathStart offset computed from the parsed parts so it mirrors urlSerialize's prefix emission exactly — scheme: plus, with a host present, // + non-empty userinfo (user[:pw]@) + host + (:port when set), plus the /. sentinel for a non-opaque //-led path. Empty/invalid input and a null slice return 0. Context-free: no realm, no exception channel.

Validation:

• Compiled runtime fixture 313 → 314/314: fixed-offset cases (userinfo+port http://user:pw@example.com:8080 → 31, default-port elision → 18, file: empty-authority → 7, opaque mailto: → 7), JS-derived parity pathStart === href.length - pathname.length for the non-special hierarchical (foo://host:9/p) and /.-sentinel (foo:////p) cases, origin.length getter parity for https, and invalid/empty/null → 0.
• bun-private-abi-audit: implemented-private 304/421 (pending 117). c-api-audit: 320 private profile exports, 117/117 public complete.
• Worktree A/B at base bcf9b0fd (isolated from unrelated uncommitted wasm work in the shared checkout): baseline 1152 passed / 3 failed vs candidate 1152 passed / 3 failed — identical failure sets (the three known pre-existing: gc slab size, wasm.decode malformed, wasm.validate GC canonical recursive type identity). Zero new failures.

Inventories refreshed: Home 311/431 implemented (120 pending), Bun 304/421 (117 pending). Docs boundary evidence published in docs/abi/README.md. Only the DOMURL object-creation group remains in the URL cluster.