fix(tools): single ordered driver lane for the sub-dispatch scheduler; validate the cap

Responding to ds-review-bot round 2 on #658 (three critical findings, one
warning — all rooted in the pump/commit split racing ordered stages):

- ONE driver lane now owns every ordered stage: the start append, prepare
  (pre-execute/guards), and the head-of-line commit (post-execute, context
  deferral, settle append). start() is awaited before the next entry can
  start, so concurrent submissions can no longer run pre-execute pipelines
  concurrently; only the around-dispatch/body stage overlaps, matching the
  native loop's fillPool sequencing.
- An exclusive call's barrier now holds through its COMMIT: later starts
  wait for the exclusive pipeline (post-execute included) to finish, the
  native exclusive-group semantics.
- drainDispatches() awaits the driver run itself, so a commit already
  mid-flight when the program returns is drained before run_code closes
  the turn — the settle event and deferred contexts land inside it.
- maxParallelSubCalls is resolved and validated at construction (positive
  integer), so direct construction can no longer wedge the pool with 0.

New tests: overlapping-submission ordered-prepare, barrier-through-commit,
drain-mid-commit, cap rejection. 96 keyless snapshots replay unchanged;
Agent Note updated (both languages).

packages/core/tools/src/index.ts

@@ -635,6 +635,15 @@ interface FusedToolSignal {
   dispose(): void
 }
 
+/** Resolve the run_code overlap cap at the owning config boundary (direct construction bypasses the Loader schema). */
+function resolveMaxParallelSubCalls(value: number | undefined): number {
+  const maxParallelSubCalls = value ?? 10
+  if (!Number.isInteger(maxParallelSubCalls) || maxParallelSubCalls < 1) {
+    throw new Error('maxParallelSubCalls must be a positive integer')
+  }
+  return maxParallelSubCalls
+}
+
 /**
  * Tool registry and execution pipeline. Scoped registrations shadow globals;
  * one visibility resolver feeds presentation, lookup, and dispatch.
@@ -681,7 +690,7 @@ export class ToolRegistry extends Service {
     // the filterable global/scoped capability layers.
     this.codeTransport = this.mode === 'native'
       ? undefined
-      : createRunCodeTool(this, () => this.requireCodeRuntime(), config.maxParallelSubCalls ?? 10)
+      : createRunCodeTool(this, () => this.requireCodeRuntime(), resolveMaxParallelSubCalls(config.maxParallelSubCalls))
     ctx.systemPrompt.tools(context => this.wireSchemas(context.scope))
     if (this.mode !== 'native') {
       ctx.systemPrompt.section({