fix: harden event store, serialization, and grant engine; wire router chat

Event log integrity (sole source of truth):
- SqliteEventStore: serialize append/appendAll under a Mutex, enable WAL +
  busy_timeout + synchronous=NORMAL, roll back on any Throwable. Replaces the
  app-computed `SELECT MAX(seq)+1` read-modify-write that dropped events under
  concurrent appends (race was invisible to CI: only the in-memory store and
  single-threaded :memory: tests were ever exercised).
- Serialization: encodeDefaults + ignoreUnknownKeys, explicit @SerialName on all
  46 EventPayload subclasses and event-referenced enum constants, @Serializable on
  RiskLevel/RiskAction. Guards the append-only log against silent corruption from
  future class renames, field removals, or default-value changes.

Approval/grant security (Invariant: approvals cannot widen authority):
- Tier authorization is now a ceiling (request.tier.level <= max granted) instead
  of set membership; SESSION grants bind to a specific toolName instead of matching
  everything; handleCreateGrant rejects empty/over-tier/scopeless grants.

Router chat wiring (Phase 0-2): ChatInput round-trip, StartChatSession from IDLE,
router-panel layout, workflows behind Ctrl+W.

Tests: SqliteEventStore concurrency, serialization round-trip + discriminator
stability + unknown-key tolerance, adversarial grant scope/tier; updated existing
approval and reducer tests for the new grant semantics and StartChatSession effect.

core/approvals/src/main/kotlin/com/correx/core/approvals/domain/DefaultApprovalEngine.kt

@@ -26,8 +26,10 @@ class DefaultApprovalEngine : ApprovalEngine {
 
         val matchingGrant = grants.firstOrNull { grant ->
             !isExpired(grant, now)
-                && scopeMatches(grant.scope, context)
-                && request.tier in grant.permittedTiers
+                && scopeMatches(grant.scope, context, request.toolName)
+                // Ceiling semantics: grant authorizes up to its highest tier, not an exact set.
+                && grant.permittedTiers.isNotEmpty()
+                && request.tier.level <= grant.permittedTiers.maxOf { it.level }
         }
 
         if (matchingGrant != null) {
@@ -83,9 +85,15 @@ class DefaultApprovalEngine : ApprovalEngine {
     private fun isExpired(grant: ApprovalGrant, now: Instant): Boolean =
         grant.expiresAt != null && grant.expiresAt <= now
 
-    private fun scopeMatches(scope: GrantScope, context: ApprovalContext): Boolean = when (scope) {
-        is GrantScope.SESSION  -> true
-        is GrantScope.STAGE    -> context.identity.stageId == scope.stageId
-        is GrantScope.PROJECT  -> context.identity.projectId == scope.projectId
-    }
+    private fun scopeMatches(scope: GrantScope, context: ApprovalContext, requestToolName: String?): Boolean =
+        when (scope) {
+            // SESSION grants are scoped to a specific tool name.
+            // A null toolName on either side means the binding is absent; treat as no-match
+            // to prevent a legacy/malformed grant from becoming a blanket approval.
+            is GrantScope.SESSION -> scope.toolName != null
+                && requestToolName != null
+                && scope.toolName == requestToolName
+            is GrantScope.STAGE   -> context.identity.stageId == scope.stageId
+            is GrantScope.PROJECT -> context.identity.projectId == scope.projectId
+        }
 }

core/approvals/src/test/kotlin/com/correx/core/approvals/DefaultApprovalReducerTest.kt

@@ -68,7 +68,7 @@ class DefaultApprovalReducerTest {
         val grantId = GrantId("grant-1")
         val payload = ApprovalGrantCreatedEvent(
             grantId = grantId,
-            scope = GrantScope.SESSION,
+            scope = GrantScope.SESSION(),
             permittedTiers = setOf(Tier.T1, Tier.T2),
             reason = "user approved",
             expiresAt = null,
@@ -81,7 +81,7 @@ class DefaultApprovalReducerTest {
 
         assertTrue(state.grants.containsKey(grantId))
         assertEquals(grantId, state.grants[grantId]?.id)
-        assertEquals(GrantScope.SESSION, state.grants[grantId]?.scope)
+        assertEquals(GrantScope.SESSION(), state.grants[grantId]?.scope)
     }
 
     @Test
@@ -90,7 +90,7 @@ class DefaultApprovalReducerTest {
 
         val addPayload = ApprovalGrantCreatedEvent(
             grantId = grantId,
-            scope = GrantScope.SESSION,
+            scope = GrantScope.SESSION(),
             permittedTiers = setOf(Tier.T1),
             reason = "granted",
             expiresAt = null,

core/approvals/src/test/kotlin/com/correx/core/approvals/DefaultApprovalRepositoryTest.kt

@@ -76,7 +76,7 @@ class DefaultApprovalRepositoryTest {
                 metadata = metadata(sessionId, "event-1"),
                 payload = ApprovalGrantCreatedEvent(
                     grantId = grantId,
-                    scope = GrantScope.SESSION,
+                    scope = GrantScope.SESSION(),
                     permittedTiers = setOf(Tier.T1, Tier.T2),
                     reason = "approved",
                     expiresAt = null,