fix(hyperframes-media): surface the real reason a TTS line failed [P2]

[miguel-heygen]

Problem

synthesizeHeygen() swallowed every failure into a bare { ok: false }:

• a thrown HTTP error (e.g. 402 plan_upgrade_required from heygenJSON) was caught and discarded by a bare catch {},
• a missing audio_url, a failed audio fetch, and a failed transcode each returned { ok:false } with no reason.

audio.mjs then logged line N: TTS failed — omitted for every line with zero diagnostic detail, so the actual cause (a plan/billing error, an auth problem, etc.) took a hand-rolled repro script to find. Reported from real usage: "surface e.message in the anomaly text."

Fix

Every failure path now returns an { error } string stating why — the caught exception message, the HTTP status, the missing audio_url, or the failed transcode stage — and audio.mjs appends it to the anomaly (TTS failed — omitted (<error>)). The subprocess providers (elevenlabs / kokoro) get the same treatment via a shared synthResult() helper (naming the non-zero exit or the missing wav).

synthesizeHeygen takes an optional injectable deps arg (defaults to the real network/ffmpeg impls) purely so the failure paths are unit-testable.

Verification

tts.test.mjs (node:test) — 4 new tests, 9 total pass: a thrown 402 surfaces in error (contains 402 + plan_upgrade_required); a non-ok audio fetch surfaces its HTTP status; a missing audio_url is named; synthResult names a non-zero subprocess exit. No behavior change on the success path.