test: guard consumer-side verdict maps stay total over the enum

[ericckzhou]

Summary:

• Adds tests/meta/test_verdict_consumer_coverage.py, an anti-entropy guard that asserts the two consumer-side tables which turn a Verdict into a CI signal stay total over the enum.
• render._EXIT_CODES must cover every Verdict — exit_code_for indexes it directly (_EXIT_CODES[verdict]), so a verdict missing from the map is a KeyError crash the moment that verdict reaches the CLI. The contract is "one exit code your CI can gate on"; a crash is not an exit code.
• Every verdict must map to a documented verdict-derived code (0/1/2/4); codes 3/5/6 belong to non-verdict surfaces (errors, diff) and a verdict must never resolve to one.
• diff._QUALITY_RANK must partition the enum into ranked verdicts plus exactly the two deliberately off-ladder ones (INVALID_EVAL, INSUFFICIENT). _classify_transition reads the rank with .get(), so a new verdict missing from the map silently becomes OTHER_CHANGE and suppresses a regression signal — this forces a conscious rank-vs-off-ladder decision instead.

Validation:

• uv run pytest tests/meta tests/unit -q → 720 passed.
• Mutation check (in-process, no files touched): popping one verdict from each consumer map makes all three new assertions fail with actionable messages — confirms the guard bites and is not vacuously passing.
• uv run ruff check / ruff format --check clean on the new file.

Notes:

• Tests-only; no runtime behavior changes.
• Closes the seam left open by the existing producer-side guard (test_resolver_branch_count.py) and the frozen-enum guard (test_verdict_models.py): the enum could grow and the resolver emit a new class while a downstream consumer map went silently out of sync, surfacing only in a user's CI.