Consolidate signals-coverage.test.ts into the signals main suites

[JSONbored]

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

Three files totaling ~6.2k lines all test src/signals/engine (two also cover src/signals/reward-risk):

• test/unit/signals.test.ts (1506 lines, describe("world-class backend signals"))
• test/unit/signals-v2.test.ts (2241 lines, describe("v2 signal builders"))
• test/unit/signals-coverage.test.ts (2508 lines, describe("signal coverage edge cases"))

Top-level describe titles don't literally collide, so this needs a real overlap audit rather than a blind merge — the naming (v2, coverage) reads as organic accretion.

Task

• Map which exported builders/functions each file covers; identify cases in signals-coverage.test.ts that duplicate branch arms already covered in the other two.
• Move genuinely unique edge cases into whichever main suite owns that builder; drop duplicates.
• Delete signals-coverage.test.ts (or, if a substantial unique core remains, rename it to describe the actual concern — not "coverage").
• Verify zero coverage regression on src/signals/engine.ts + src/signals/reward-risk.ts with an unsharded npm run test:coverage before/after comparison.

[linear-code]

JSO-5941