fix(aws-lambda): validate event S3 URIs against render bucket (F-004) (#1213)

## Summary

- Adds `validateEventS3Uris()`, called immediately after `unwrapEvent()` in the Lambda handler before any S3 I/O.
- If `HYPERFRAMES_RENDER_BUCKET` env var is set, every S3 URI in the event (`ProjectS3Uri`, `PlanOutputS3Prefix`, `PlanS3Uri`, `ChunkOutputS3Prefix`, `ChunkS3Uris`, `AudioS3Uri`, `OutputS3Uri`) must resolve to that bucket. Mismatches throw `S3_URI_NOT_ALLOWED`.
- Env var unset → validation skips (backwards-compatible; existing deployments without the var continue to work).
- CDK stack (`HyperframesRenderStack`) auto-wires `HYPERFRAMES_RENDER_BUCKET: this.bucket.bucketName` so new deployments are protected without manual config.
- `S3_URI_NOT_ALLOWED` added to all three `NON_RETRYABLE_*` lists in the Step Functions state machine so the state machine does not retry on this error.

## Security

**F-004 MED** — The Lambda handler accepted S3 URIs from the event payload without verifying they targeted the function's own render bucket. An attacker who could inject a crafted Step Functions execution input could route `GetObject` / `PutObject` calls to arbitrary buckets in the same AWS account, potentially exfiltrating plan data or overwriting objects in unrelated buckets.

## Test plan

- [x] `handler` rejects a `plan` event whose `ProjectS3Uri` targets a different bucket — `S3_URI_NOT_ALLOWED` thrown, zero S3 ops recorded
- [x] `handler` rejects an `assemble` event with one cross-bucket chunk URI
- [x] Validation is skipped when `HYPERFRAMES_RENDER_BUCKET` is unset (no regression for existing callers)
- [x] All 12 handler unit tests pass

packages/aws-lambda/src/cdk/HyperframesRenderStack.ts

@@ -110,6 +110,7 @@ export class HyperframesRenderStack extends Construct {
         NODE_OPTIONS: "--enable-source-maps",
         TMPDIR: "/tmp",
         HYPERFRAMES_LAMBDA_CHROME_SOURCE: chromeSource,
+        HYPERFRAMES_RENDER_BUCKET: this.bucket.bucketName,
       },
     });
 
@@ -195,6 +196,7 @@ export class HyperframesRenderStack extends Construct {
     const NON_RETRYABLE_PLAN = [
       "FFMPEG_VERSION_MISMATCH",
       "PLAN_HASH_MISMATCH",
+      "S3_URI_NOT_ALLOWED",
       "BROWSER_GPU_NOT_SOFTWARE",
       "FONT_FETCH_FAILED",
       "PLAN_TOO_LARGE",
@@ -204,12 +206,14 @@ export class HyperframesRenderStack extends Construct {
     const NON_RETRYABLE_CHUNK = [
       "FFMPEG_VERSION_MISMATCH",
       "PLAN_HASH_MISMATCH",
+      "S3_URI_NOT_ALLOWED",
       "BROWSER_GPU_NOT_SOFTWARE",
       "ChromeBinaryUnavailableError",
     ];
     const NON_RETRYABLE_ASSEMBLE = [
       "FFMPEG_VERSION_MISMATCH",
       "PLAN_HASH_MISMATCH",
+      "S3_URI_NOT_ALLOWED",
       "FORMAT_NOT_SUPPORTED_IN_DISTRIBUTED",
       "ChromeBinaryUnavailableError",
     ];