chore(engine): bump to 3.15.4 for the dead type-import removal in gate-advisory.ts

check-engine-parity holds the two hand-duplicated gate-decision twins
(packages/loopover-engine/src/advisory/gate-advisory.ts and src/rules/advisory.ts)
in lockstep: touching one without the other requires an engine version bump. That is
the mechanism, and it is doing its job here.

the engine twin. They are genuinely dead there and NOT in the host: the engine copy is
a deliberately slimmed re-implementation (#4881) that omits buildIssueAdvisory /
addIssueFindings / collisionClustersForPull, which are what use those types on the host
side. So there is no matching host edit to make -- the asymmetry is correct, and the
version bump is the sanctioned way to record it.

No behaviour change: type-only imports are erased at compile time. The bump exists so
the parity contract stays enforceable, not because the gate decides anything
differently. packages/loopover-miner/expected-engine.version moves in lockstep, as its
own check requires.

Author: JSONbored

packages/loopover-engine/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@loopover/engine",
-  "version": "3.16.0",
+  "version": "3.16.1",
   "license": "AGPL-3.0-only",
   "type": "module",
   "description": "Shared deterministic engine logic for the LoopOver review stack and loopover-miner.",

packages/loopover-miner/expected-engine.version

@@ -1 +1 @@
-3.16.0
+3.16.1