test(game): coverage gate for decomposed Game modules (Phase 8 of #134)

[nodots]

Phase 8 of the epic (#133) — coverage gate (#134)

Stacked on #142 (Phase 7). Retarget to development as parents merge.

Adds targeted tests for the decomposed Game/ modules plus a COVERAGE=1-gated threshold scoped to src/Game/.

Coverage added

Module	Before	After (stmt/branch)
guards.ts	27%	100% / 100%
robot.ts	25%	88% / 88%
cube.ts	88% / 70%	93% / 76%
facade index.ts	62% / 42%	81% / 61%
turnFlow.ts	44% / 30%	76% / 66%

src/Game/ aggregate: ~59% → 80.5% stmts, 71.8% branch, 89% funcs, 82.5% lines.

The gate

coverageThreshold: {
  './src/Game/': { statements: 75, functions: 75, lines: 75, branches: 70 },
}

• Directory-path key → aggregate across src/Game/ (not per-file), so well-covered modules balance the thin executeRobotTurn delegator and the large turnFlow.
• Enforced only under COVERAGE=1 (the CI coverage job); normal jest runs are unaffected.
• Branch floor is 70 (just under the current 71.8% aggregate) and ratchets toward 75 as the remaining turnFlow no-move/blocked paths and the gnuPositionId getter gain coverage — per the "ratchet up, don't jump" plan.

Verification

• COVERAGE=1 npx jest: exit 0, gate satisfied
• npx jest: 518 passed / 12 skipped, exit 0

Follow-up

Branch → 75 needs blocked-board fixtures for roll's no-move transitions and a new Game() fixture for the gnuPositionId getter; deferred so the gate can land now.

Refs #134