Fold reward-risk-engine-branch-coverage.test.ts into the reward-risk main suites

[JSONbored]

Part of the '-coverage' bolt-on consolidation epic (#8574 — non-closing reference).

test/unit/reward-risk-engine-branch-coverage.test.ts (171 lines) is a bolt-on from the #2281 engine lift, covering tie-break/defensive branches of src/signals/reward-risk + src/signals/engine. Sibling suites already exist: reward-risk-freshness.test.ts, reward-risk-reports.test.ts, reward-risk-competition-fail-closed.test.ts.

Task

• Move each case into whichever sibling suite owns the function under test (freshness internals → reward-risk-freshness.test.ts, etc.), retitling to behavior specs.
• Delete reward-risk-engine-branch-coverage.test.ts.
• Verify zero coverage regression on src/signals/reward-risk.ts with an unsharded npm run test:coverage before/after comparison.

Coordinate with the signals consolidation (#8576 — non-closing reference) if both are in flight; they touch adjacent files but should stay separate PRs.

[linear-code]

JSO-5943