enh(test-platform): add branch-coverage floor and promote no-source-grep on tests/**

[trek-e]

Problem

Two test-rigor gates are weaker than ADR-456 intends:

1. No branch-coverage floor. c8 enforces line floors only (lib ~70%, scripts ~55%). Untested branches in the high-risk UNMUTATED modules (state/phase/verify/init) are invisible.
2. no-source-grep is warn and not applied to tests/**. Source-text-grep test smells slip in (the audit redesigns removed several: install Kilo block, sh-hook-paths, plan-review-convergence).

Action

1. Add a c8 --branches floor (start ~60 global) plus a stricter per-file branch floor for the high-risk UNMUTATED modules.
2. Promote no-source-grep to error and apply it to tests/**, splitting allowed product-/config-file reads (.md/.json/.yml) from blocked source-file grep (.cjs/.ts internal symbols). Fix any surfaced violations behaviorally.

Note: a complementary allow-test-rule tracking gate (requires #NNN on new exemptions) already landed during the audit; promoting no-source-grep makes that gate meaningful on tests/**.

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Surfaced by the 2026-06-13 QA test-architecture audit of the ADR portfolio.

[trek-e]

This was generated by AI during triage.

Agent Brief

Category: enhancement (test platform)
Summary: Add a branch-coverage floor to the coverage gate and promote the no-source-grep lint rule onto tests/**.

Current behavior:

• The coverage gate checks only --lines (no --branches floor anywhere in the c8 invocations), so branch coverage can regress unchecked.
• The local/no-source-grep rule is entirely absent from the tests/** ESLint block (it exists only on source/scripts globs, and there only at warn), so new tests can still ship readFileSync + .includes() source-grep assertions.

Desired behavior:

• A branch-coverage floor is enforced in the coverage gate, set at or just below the current measured branch percentage to avoid a false-red CI introduction.
• no-source-grep is active on tests/** — first as warn to surface existing violations, then promoted to error once they're rewritten behaviorally.

Key interfaces:

• The coverage scripts — add a --branches <n> floor (measure actual branch % first, then set the floor).
• The tests/** ESLint config block — add local/no-source-grep; rewrite surfaced violations as behavioral tests (call exported fn, assert typed fields), then escalate to error.

Acceptance criteria:

• The coverage gate fails when branch coverage drops below the established floor.
• The floor is set from a measured baseline (document the measured % in the PR), not an aspirational number that red-lights CI on landing.
• no-source-grep is enforced on tests/**; all surfaced violations are rewritten behaviorally and the rule is at error.

Out of scope:

• Per-file/per-module stricter floors for the currently-unmutated module set (note them as a follow-up if not done here).
• Changing the existing line-coverage thresholds.