feat(research): add frozen same-dimension composition comparator

[Pigbibi]

Summary

• Add an independent, pure research comparator for the existing same-dimension-min candidate: preserve L/R dimensions, exact input deduplication, full source identity, validity gates, and canonical decision digests
• Freeze the narrow synthetic trial interface and keep DESIGN_ONLY_NOT_RUNTIME; output execution, risk-increase, and release authority always remain false
• Add 26 synthetic tests, including legacy priority characterization, .1 intersection versus .05 multiplication, L/R algebra, stale required inputs, opportunity veto, source/hash/scope/unit failures, complete Git revision lengths, and inapplicable input integrity

Scope

Exactly three files. No legacy arbiter, runner, consumer, provider, data, configuration, dependency pin, or workflow changes. No provider/model/broker requests, production run, historical backtest, return claim, PIT/OOS claim, or consumer-adoption claim. Strong release/state/intent integration remains separate work.

Validation

• New research tests: 26 passed; root independently reran all 26 with network and external-process access blocked
• Focused helper + legacy arbiter + V2 envelope suite: 65 passed, 52 subtests passed; external network attempts: 0
• Local full suite: 432 passed, 3 failed, 76 subtests passed. The same three missing-ai_gateway_client failures occur on the unchanged baseline (406 passed, 3 failed); existing CI must verify the declared AI extra in its normal dependency environment
• Full-repository Ruff 0.16.10 passed. The repository pins 0.16.9; the pin is unchanged and CI remains authoritative for that version
• Exact patch application and whitespace checks passed
• Package build was not run locally because the build dependency is unavailable; existing CI includes the package build
• Review fixes have RED→GREEN coverage; no live or consumer integration was performed

Base checked immediately before publication: 5671fa45c6ab373bff79141dab1a9d7b18f59f97.