feat(869-c4): enforcement/activation version-coupling assertion

Part of #869 (Point-1 C4). Locks the lease broker's ENFORCEMENT half
(launch-runtime.py) and ACTIVATION half (execLeaseGatedRuntime, C1's
LEASE_ACTIVATION_CAPABILITY) as one versioned unit, closing the #828
version-skew gap C1 only made observable.

- New packages/mosaic/framework/tools/lease-broker/activation_version_gate.py:
  EXPECTED_ACTIVATION_CAPABILITY (enforcement-owned, mirrors but is
  independent from C1's LEASE_ACTIVATION_CAPABILITY), a fail-closed
  subprocess probe of the CLI's hidden `mosaic __lease-capability`
  command, and an assertion that FAILS LOUD (VersionCouplingError with
  an actionable #869 remediation message) on mismatch OR absence —
  never a silent pass, never a dead gate.
- launch-runtime.py now runs this assertion first, before any broker
  registration, and denies with a dedicated exit code (65) distinct
  from its existing usage (64) and registration-failure (1) codes.
- Red-first tests: src/mutator-gate/version_coupling_unittest.py
  (module-level match/mismatch/name-mismatch/absent-capability cases,
  and seam-level LAUNCHER.main() cases proving the gate runs before
  broker registration and never silently passes).
- Existing fail-closed-on-absent-identity lock
  (runtime_tools_unittest.py) stays green: matching-capability fakes
  were injected into its pre-existing LAUNCHER.main() calls so the new
  gate runs alongside, not in place of, identity enforcement.
- mutator-gate.acceptance.spec.ts updated to supply a fake CLI-capability
  probe (matching the existing fake-broker/fake-runtime-binary test
  doubles already in that suite) everywhere it drives launch-runtime.py
  as a real subprocess.

launch.ts and lease-activation-probe.ts are untouched (C2/C5 avoidance,
C1 read-only).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/mosaic/package.json

@@ -25,7 +25,7 @@
     "lint": "eslint src",
     "typecheck": "tsc --noEmit",
     "test": "vitest run --passWithNoTests && pnpm run test:framework-shell",
-    "test:framework-shell": "python3 src/lease-broker/daemon_deadline_unittest.py && python3 src/lease-broker/normative_fragments_unittest.py && python3 src/lease-broker/receipt_challenge_unittest.py && python3 src/lease-broker/context_recovery_unittest.py && python3 src/lease-broker/recovery_runtime_unittest.py && python3 src/lease-broker/recovery_b1_adversarial_unittest.py && python3 src/lease-broker/framework_skill_portability_unittest.py && python3 src/mutator-gate/runtime_tools_unittest.py && python3 src/mutator-gate/runtime_launch_guard_unittest.py && python3 framework/tools/lease-broker/check-runtime-launches.py --root ../.. && bash framework/tools/codex/test-pr-diff-context.sh && bash framework/tools/qa/test-deps-preflight.sh && bash framework/tools/git/test-pr-review-gitea-comment.sh && bash framework/tools/git/test-ci-queue-wait-branch-absent.sh && bash framework/tools/git/test-git-credential-mosaic.sh && bash framework/tools/git/test-gitea-token-identity.sh && bash framework/tools/_scripts/test-install-ordering-guard.sh"
+    "test:framework-shell": "python3 src/lease-broker/daemon_deadline_unittest.py && python3 src/lease-broker/normative_fragments_unittest.py && python3 src/lease-broker/receipt_challenge_unittest.py && python3 src/lease-broker/context_recovery_unittest.py && python3 src/lease-broker/recovery_runtime_unittest.py && python3 src/lease-broker/recovery_b1_adversarial_unittest.py && python3 src/lease-broker/framework_skill_portability_unittest.py && python3 src/mutator-gate/runtime_tools_unittest.py && python3 src/mutator-gate/runtime_launch_guard_unittest.py && python3 src/mutator-gate/version_coupling_unittest.py && python3 framework/tools/lease-broker/check-runtime-launches.py --root ../.. && bash framework/tools/codex/test-pr-diff-context.sh && bash framework/tools/qa/test-deps-preflight.sh && bash framework/tools/git/test-pr-review-gitea-comment.sh && bash framework/tools/git/test-ci-queue-wait-branch-absent.sh && bash framework/tools/git/test-git-credential-mosaic.sh && bash framework/tools/git/test-gitea-token-identity.sh && bash framework/tools/_scripts/test-install-ordering-guard.sh"
   },
   "dependencies": {
     "@mosaicstack/brain": "workspace:*",