Add TAS bot session abstraction for deterministic gameplay exploration

Introduces runtime/testing/tas.ts with three key abstractions:
- inputSequence(): fluent DSL for building frame-by-frame input sequences
- createTASSession(): stateful session with step/checkpoint/branch/search
- serializeRecording/deserializeRecording: recording persistence

34 new tests covering input builder, session stepping, checkpoints,
branching, BFS goal search, and end-to-end TAS workflows.

https://claude.ai/code/session_01Nkfpuox6vhiCzp9Bh3mwZe

Author: claude

runtime/testing/index.ts

@@ -73,6 +73,21 @@ export type {
   PropertyResult,
 } from "./property.ts";
 
+// TAS (Tool-Assisted Speedrun) session
+export {
+  inputSequence,
+  createTASSession,
+  serializeRecording,
+  deserializeRecording,
+} from "./tas.ts";
+export type {
+  InputSequenceBuilder,
+  Checkpoint,
+  TASSessionOptions,
+  GoalSearchOptions,
+  TASSession,
+} from "./tas.ts";
+
 // Draw call capture & visual assertions
 export {
   enableDrawCallCapture,