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.

#9553's sweep removed two dead TYPE-only imports (IssueRecord, CollisionReport) from
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": "3.15.3",
+  "version": "3.15.4",
   "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 @@
-3.15.3
+3.15.4