ABI: implement exact Yarr RegularExpression boundary

Parent: #134 Profiles: #163, #164 Dependencies completed: zig-utils/zig-regex#11, zig-utils/zig-regex#12

Implement the six native Yarr__RegularExpression__* imports used by the pinned Home/Bun JavaScriptCore corpus:

• init
• deinit
• isValid
• matchedLength
• matches
• searchRev

Acceptance criteria:

• Match the real C++/Rust executable ABI, including the two-argument searchRev(RegularExpression*, BunString) signature; document that the checked-in Zig declaration is stale.
• Compile BunString patterns through zig-regex with JavaScript-compatible ignore-case, multiline, dot-all, Unicode, UnicodeSets, and sticky behavior where applicable.
• Preserve valid/invalid compiled state and JavaScriptCore last-match-length behavior, including reset to -1 after a failed match.
• Return UTF-16 code-unit positions and lengths, including astral and lone-surrogate input.
• Match JavaScriptCore reverse-search behavior for overlap, empty matches, and subset matches.
• Keep allocation, null-input, and object lifetime behavior deterministic at the native boundary.
• Add focused ABI tests plus a linked consumer fixture for all six imports.
• Refresh both private-import inventories, the implementation matrix, README status, and parent tracking issues with exact evidence.

Backward compatibility is intentionally out of scope while zig-js is pre-release and unused.

Implementation: bc4b9e32 Documentation/inventories: ff6d8730 zig-regex prerequisites: fd4927c, 86159c5

Completed on origin/main.

Evidence:

• focused Yarr test: 3/3 selected harness tests passed, 0 leaked;
• compile-link-runtime fixture: 184/184 symbols linked and runtime matrix passed;
• Home audit: 185/431 implemented, 246 pending;
• Bun audit: 179/421 implemented, 242 pending;
• public audit: 117/117 public, 19 extensions, 189 private/support exports;
• batched build/audit gate: 12/12 steps succeeded.

The README retains the current July 17 isolated 1,540-sample JSC benchmark report unchanged because this ABI-only slice does not alter or rerun benchmarked engine paths.