fix(producer): scope and harden the markup-payload sniff

Review follow-up on the STUDIO-5433 defense.

Correctness

- The sniff ran above the documented video/audio failure split, so an
  <audio> src that resolved to an HTML payload aborted the whole render
  instead of degrading to duration 0. It now runs inside the same try, so
  video surfaces the typed error while audio still drops out, with a
  warning naming the element.
- Raw fs errors (EISDIR on a directory src, EACCES, the existsSync->open
  ENOENT race, EMFILE) escaped and failed the compile with an unclassified
  error carrying an unredacted temp path. The sniff is now a classifier that
  never throws: an unreadable file reports "not markup" and the real probe
  produces the real error.
- Elements whose duration the compiler never resolves (a data-end video, a
  looping audio) skipped the sniff entirely, so the original ffprobe error
  still escaped, and looping audio was reported as owner "system" after
  every frame had been captured. Video is now caught in the asset preflight,
  which sees every local src regardless of authored timing; audio is
  classified per-element in audioMixer as source/invalid_media/owner "user",
  keeping audio failures non-fatal as they already were.
- Detection is a byte-level check for a leading "<" (BOM-, whitespace- and
  NUL-tolerant, looped read) instead of a <!doctype|<html|<?xml string
  prefix, which missed a NUL-prefixed payload, >256B of leading whitespace,
  UTF-16-encoded HTML, and a prolog-less <svg. No supported container starts
  with "<", so the allowlist no longer grows per payload shape.
- finally { await fh.close() } could replace the in-flight typed error with
  the close error.

Routing and privacy

- MARKUP_NOT_MEDIA is now in SAFE_RENDER_ERROR_CODES, the Lambda terminal
  name map, the CDK and SAM non-retryable plan lists, and the Cloud Run
  non-retryable set, and the class carries owner/retryable. Previously the
  API emitted errorCode: undefined and a deterministic authoring bug burned
  the full distributed retry budget.
- The message no longer carries 32 raw payload bytes or the src.
  redactTelemetryString preserves host and path for HTTP srcs, so
  per-tenant CDN paths reached a message the server forwards to clients.
  Correlation is a sha256 element fingerprint, matching
  AssetMediaTypeMismatchError.
- The message names both causes (unresolved nested-composition URL, or an
  HTML/XML error page served as 200) rather than misdiagnosing an S3 403
  body as an authoring bug.

Tests

- Byte-level detection is unit-tested in engine: markup shapes, BOMs,
  UTF-16, nine container signatures, unreadable inputs.
- Replaced the tautological assertions. The old checks for "html" in and
  "moov" absent from a fixed message template could not fail for any input.
- New coverage for audio degradation, the audioMixer classification, the
  preflight video/image/audio split, and the API error metadata.
- The sibling htmlCompiler.mediaType failure was a vitest-under-bun runner
  mismatch, not a missing ffmpeg binary. It passes, including the 4-wide
  probe-semaphore invariant the sniff now runs inside.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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

@@ -78,6 +78,8 @@ const EXPECTED_NON_RETRYABLE_ERRORS = new Set([
   "PLAN_V2_INTEGRITY_UNRECOVERABLE",
   "VIDEO_SOURCE_UNRENDERABLE",
   "INVALID_VIDEO_METADATA",
+  "MARKUP_NOT_MEDIA",
+  "MarkupNotMediaError",
   "PlanV2IntegrityError",
   "PLAN_ARTIFACT_DIGEST_MISMATCH",
   "FORMAT_NOT_SUPPORTED_IN_DISTRIBUTED",