feat(tts): map ratatoskr:sindra -> miranda voice

Sindra now voices with the dots "miranda" voice (operator-directed);
donut stays on "donut", other agents fall to the "glados" default.
One entry in _TTS_VOICE_MAP + a test; contract DEC-8 updated.
Live-verified on :8765 (/api/tts with agent_id=ratatoskr:sindra -> 200
audio/wav @ 48kHz).

docs/contracts/donut_voiced_interview.contract.md

@@ -174,7 +174,8 @@ each independently shippable. Slice order is chosen for fastest visible result.
   dots clones a voice server-side from a reference clip + transcript; the client just
   passes a voice NAME (GET /v1/voices lists them: donut/glados/emmie/miranda). The
   `donut` voice carries over from chatterbox, so `_TTS_VOICE_MAP` maps
-  `ratatoskr:donut → "donut"` unchanged. NOTE the case: lowercase `"donut"` (Zonos used
+  `ratatoskr:donut → "donut"`; `ratatoskr:sindra → "miranda"` (operator-directed
+  2026-08-10). NOTE the case: lowercase `"donut"` (Zonos used
   `"Donut"`). Non-interview agents fall to the dots default `"glados"` (was chatterbox
   `"glados_25s"` / Zonos `"Cora"`, neither of which exists on dots). New voices are a
   one-line request to infra-ops (derived from the canonical voice corpus).