feat(validation): auto build-gate on terminal freestyle stage (Gate 4 floor)

Closes the COMPLETED-lie where a freestyle stage passes by producing a
schema-valid artifact regardless of whether its build actually succeeded:
a verify stage could run 'npm run build', watch it fail, then self-attest a
build_verified artifact and transition to done. Tool failures never failed
the stage.

The compiler now flags the terminal stage autoBuildGate=true when no stage
declares an explicit build_expectation. Code-ness is decided at runtime, not
compile time (the declared kind is always the generic file_written; only the
written paths reveal code): runExecutionGate folds the FileWritten manifest via
sessionProducedBuildTarget() and promotes to a PROJECT build when a build
manifest (package_json/gradle_module) or an imports_resolve-bearing path was
written, then runs the real command from .correx/project.toml [commands]. A
non-clean exit fails the stage retryably; a docs-only plan is left untouched.

Live-proven (run 3, session 28812b44): gate fired on the real
'npm --prefix frontend run build', exit 2 on a bad tsconfig, RetryAttempted
instead of a false COMPLETED.

Modules green: transitions+workflow 67, kernel 60.

core/kernel/src/main/kotlin/com/correx/core/kernel/orchestration/SessionOrchestrator.kt

@@ -137,6 +137,7 @@ import com.correx.core.tools.registry.ToolRegistry
 import com.correx.core.transitions.evaluation.EvaluationContext
 import com.correx.core.transitions.evaluation.PromptResolver
 import com.correx.core.transitions.execution.StageExecutionResult
+import com.correx.core.transitions.graph.BuildExpectation
 import com.correx.core.transitions.graph.StageConfig
 import com.correx.core.transitions.graph.WorkflowGraph
 import com.correx.core.transitions.resolution.TransitionDecision
@@ -237,6 +238,11 @@ private const val REVIEW_OBJECTIVE_CAP = 4_000
 private const val STATIC_ANALYSIS_SUMMARY_CAP = 2_000
 private const val STATIC_ANALYSIS_FEEDBACK_CAP = 6_000
 
+// KindInference kinds for build manifests — a written file of one of these declares a buildable
+// project toolchain even before any source lands, so the auto build gate fires (see
+// SessionOrchestrator.sessionProducedBuildTarget). Kept in sync with KindInference's manifest kinds.
+private val BUILD_MANIFEST_KINDS = setOf("package_json", "gradle_module")
+
 @SuppressWarnings(
     "ForbiddenComment",
     "UnusedParameter",
@@ -2227,6 +2233,31 @@ abstract class SessionOrchestrator(
      * ToolInvocationRequested → this stage's invocation ids, FileWrittenEvent → the paths written
      * under them. Pure projection, replay-safe.
      */
+    /**
+     * True if the session has committed to a buildable project — it wrote either a real code module
+     * (a path whose inferred kind carries the `imports_resolve` COMPILER assertion: TS/React/Kotlin
+     * source) OR a build manifest ([BUILD_MANIFEST_KINDS]: package.json, a Gradle module) that
+     * declares a toolchain. Read from the recorded FileWritten manifest (invariant #9: replay-safe,
+     * no filesystem access), it is the run-time signal that warrants the auto build gate — the
+     * planner's declared kinds (uniformly `file_written`) never reveal this, but the written paths do.
+     *
+     * The manifest case is what catches the degenerate scaffold that writes only `package.json` and
+     * no sources: `npm run build` then fails for lack of an entry, so the gate rejects the empty
+     * "project" instead of letting it reach COMPLETED. A docs-only plan writes neither, so it is left
+     * alone.
+     */
+    private fun sessionProducedBuildTarget(sessionId: SessionId): Boolean =
+        eventStore.read(sessionId)
+            .mapNotNull { it.payload as? FileWrittenEvent }
+            .filter { it.postImageHash != null }
+            .map { it.path }
+            .distinct()
+            .any { path ->
+                val kind = KindInference.kindFor(path) ?: return@any false
+                kind in BUILD_MANIFEST_KINDS ||
+                    KindContractTable.assertionsFor(kind, path).any { it.id == "imports_resolve" }
+            }
+
     private fun stageWrittenPaths(sessionId: SessionId, stageId: StageId): List<String> {
         val events = eventStore.read(sessionId)
         val invocationIds = events.mapNotNull { it.payload as? ToolInvocationRequestedEvent }
@@ -2313,16 +2344,26 @@ abstract class SessionOrchestrator(
         effectives: RunEffectives,
         profileCommands: Map<String, String>,
     ): StageExecutionResult {
-        val alias = stageConfig.buildExpectation.commandAlias ?: return StageExecutionResult.Success(emptyList())
+        // An explicit build_expectation resolves directly. Otherwise, if the compiler flagged this
+        // (terminal) stage as auto-gateable and the session actually wrote a code module, promote to a
+        // PROJECT build — the LLM planner never sets build_expectation on its file_written stages, so
+        // this is the only path that gates a scaffold's compilability. Code-ness is judged here (not at
+        // compile time) from the real FileWritten manifest, so a docs-only plan is left alone.
+        val expectation: BuildExpectation? = when {
+            stageConfig.buildExpectation != BuildExpectation.NONE -> stageConfig.buildExpectation
+            stageConfig.autoBuildGate && sessionProducedBuildTarget(sessionId) -> BuildExpectation.PROJECT
+            else -> null
+        }
+        val alias = expectation?.commandAlias ?: return StageExecutionResult.Success(emptyList())
         val runner = staticAnalysisRunner
         val workspaceRoot = effectives.policy?.workspaceRoot
         if (runner == null || workspaceRoot == null) return StageExecutionResult.Success(emptyList())
         val command = profileCommands[alias]
         if (command.isNullOrBlank()) {
             log.warn(
-                "[Orchestrator] stage {} declares build_expectation={} but project profile has no '{}' " +
+                "[Orchestrator] stage {} needs a {} build gate but project profile has no '{}' " +
                     "command — skipping execution gate",
-                stageId.value, stageConfig.buildExpectation, alias,
+                stageId.value, expectation, alias,
             )
             return StageExecutionResult.Success(emptyList())
         }
@@ -2334,10 +2375,10 @@ abstract class SessionOrchestrator(
 
         log.warn(
             "[Orchestrator] execution gate failed session={} stage={} expectation={} command={}",
-            sessionId.value, stageId.value, stageConfig.buildExpectation, command,
+            sessionId.value, stageId.value, expectation, command,
         )
         return StageExecutionResult.Failure(
-            "stage ${stageId.value} did not pass its ${stageConfig.buildExpectation} build gate. " +
+            "stage ${stageId.value} did not pass its $expectation build gate. " +
                 "Fix these before proceeding:\n\n$ $command  (exit ${run.exitCode})\n" +
                 run.output.takeLast(STATIC_ANALYSIS_FEEDBACK_CAP),
             retryable = true,