feat(validation): plan-compile gate + semantic-review gate (staged verification #19/#20)

Plan-compile gate (#19): PlanCompilationCheck seam (null-on-replay),
runPlanCompileGate guards a produced execution_plan slot and compiles its
cached content via ExecutionPlanCompiler, emitting PlanCompileCheckedEvent.
On failure the architect gets a retryable hand-back and self-corrects;
exhaustion terminates the dead-end workflow instead of shipping an
uncompilable plan.

Semantic-review gate (#20): SemanticReviewer seam + StageConfig.semanticReview
opt-in (TOML semantic_review + freestyle plan JSON). runReviewGate reads the
FileWritten manifest (never blind context), asks a review-capable model for
PR-comment findings, and is advisory — blocks retryably only on FAIL plus a
correctness finding >=0.7 confidence, capped at 2 blocks so a stubborn stage
completes advisory-only rather than trapping. SemanticReviewerImpl talks to
the provider directly; any reviewer error degrades to PASS. REVIEW_MAX_TOKENS
is 8192: gemma is a reasoning model and 1024 truncated it mid-reasoning before
it reached the JSON verdict (empty content -> parse degraded to PASS).

Both gates live-QA'd on the real repo: plan-compile passed on a freestyle
web-UI scaffold; semantic review caught all 3 planted bugs in a maxOf() and
exercised the block->retry->cap safety valve end-to-end.

core/transitions/src/main/kotlin/com/correx/core/transitions/graph/StageConfig.kt

@@ -38,5 +38,10 @@ data class StageConfig(
     // FAILS the gate — catching missing files, which the runtime write-manifest alone (it only
     // inspects files that WERE written) cannot. Derived from the concrete entries of PlanStage.writes.
     val expectedFiles: List<String> = emptyList(),
+    // Opt-in to the Gate 3 semantic (LLM) reviewer for this stage (staged-verification § Gate 3). When
+    // true and a reviewer is wired, the reviewer reads this stage's produced files after the
+    // deterministic funnel passes and raises advisory PR-comment findings; a high-confidence
+    // correctness FAIL can block retryably until the review-block budget is spent. Default off.
+    val semanticReview: Boolean = false,
     val metadata: Map<String, String> = emptyMap(),
 )