Bench: version the post-V16 exact-parent evidence contract

Problem

zig build benchmark-comparison-test reaches the pinned Home tool after #630, then fails closed because tools/exact-parent-regression.ts no longer matches the SHA frozen in representative matrix V16. The integration changed after V16 for #615, #617, and #614, but the representative contract was not versioned.

Current SHA: 17e110358724b5c77202a93efda2c42635b56cbe54d722b9b90e8221fa54f1a5; V16 pin: ec52fc04d30ac1739e0561d5054692d3aac0c0a9ae2926475862ffbada018f22. The gate correctly reports V16 exact-parent integration changed without a matrix version bump.

Required implementation

• Add a frozen V17 manifest inheriting V16 without rewriting historical evidence.
• Pin the current exact-parent integration and describe the added frontend counters plus competing/transient-job and contaminated-build rejection contracts.
• Advance the validator/default manifest and structural negative test to V17.
• Keep all implemented workload families, modes, checksums, timing boundaries, external-suite rulings, and unavailable metrics unchanged.

Acceptance

• tools/test_representative_matrix.ts passes all structural cases and still rejects a mutated V17 pin.
• zig build benchmark-comparison-test passes using the pinned Home binary.
• Dependency audit and docs link checks pass.
• Flips 0 test262 cases (evidence/tooling only).

Parent: #461 Related: #614, #615, #617, #630

Completed in 9b286fea. V17 inherits every V16 scored and external-suite contract, replaces only the exact-parent integration pin, and validates the merged child so every other historical hash stays fail-closed. Evidence: structural tests 22/22, complete benchmark harness green, docs links 267/267, dependency audit green. The independent renderer-manifest failure remains tracked in #631.