engines: import f5-tts + kokoro + tortoise sidecars into the tree

The python FastAPI sidecars have lived ad-hoc at /mnt/cache/appdata/
<engine>/build/ on Lucy without version control. Bringing them into
the skald repo so the engine code travels with the cross-engine
routing it depends on.

This commit lands the VANILLA version of each engine on main:

  engines/f5-tts/    SWivid F5-TTS (CC-BY-NC weights flagged)
  engines/kokoro/    hexgrad Kokoro-82M (Apache 2.0 top to bottom)
  engines/tortoise/  neonbjb Tortoise-TTS (Apache 2.0 top to bottom)

Engine-specific kludges (question doubling, GPU coordination,
pause-duration tuning) get layered on engine/* branches per the
README. Main stays the safe-to-read baseline.

engines/README.md

@@ -0,0 +1,58 @@
+# Skald TTS engines
+
+This subtree holds the per-engine sidecars that skald's narrate path
+talks to over HTTP. Each engine has the same contract:
+
+- `POST /synthesize` — same JSON shape across engines so skald's
+  one Rust client (`skald-core::narrate::Narrator`) deserializes
+  all of them. See `engines/<name>/server.py` for the per-engine
+  implementation.
+- `GET /healthz` — boot probe + model-loaded flag.
+
+Skald routes per-request by `voices.source`: a `kokoro_*` source
+goes to `$KOKORO_URL`, a `tortoise_*` source goes to `$TORTOISE_URL`,
+anything else (`lj_speech`, generic) goes to `$F5_TTS_URL`.
+
+## Engines
+
+| Dir | Engine | License (code/weights) | VRAM | Speed | Voices |
+|---|---|---|---|---|---|
+| `f5-tts/` | SWivid F5-TTS v1 | MIT / **CC-BY-NC** | ~5GB | fast (~2x real-time on 2070S) | voice cloning (LJ Speech reference shipped) |
+| `kokoro/` | hexgrad Kokoro-82M | Apache 2.0 / Apache 2.0 | ~1GB | very fast (~50x real-time) | 50+ named presets (af_*, am_*, bf_*, bm_*) |
+| `tortoise/` | neonbjb Tortoise-TTS | Apache 2.0 / Apache 2.0 | ~5GB | **slow** (~0.014x real-time, ~74s/s of audio on 2070S, standard preset) | 26 named built-ins (lj, freeman, daniel, weaver, jlaw, etc.) |
+
+## Branch model
+
+`main` carries the **vanilla** version of each engine — what you'd
+get from a clean `pip install <engine>` plus the FastAPI sidecar
++ control-tag splitter. No engine-specific kludges. Safe to look
+at without context.
+
+`engine/<name>` branches hold engine-tuned tweaks that don't
+generalise. Examples:
+
+- `engine/kokoro` — doubled-`??` prosody hack for the 82M's weak
+  question intonation, paragraph/scene/breath gap durations tuned
+  for af_heart's pacing, notes on how respellings need to be all-
+  lowercase to avoid letter-by-letter spell-out by misaki.
+- `engine/tortoise` — GPU exclusivity coordinator (stops F5 +
+  Kokoro before a Tortoise run since the 2070 Super can't host
+  all three at once), preset choice ergonomics, character→tortoise-
+  voice seed assignments.
+
+When deploying an engine to Lucy, the build dir at
+`/mnt/cache/appdata/<engine>/build/` tracks the engine's branch:
+
+```bash
+cd /mnt/cache/appdata/kokoro/build
+git fetch && git checkout engine/kokoro
+docker compose -p <name> up -d --build
+```
+
+## GPU coordination (2070 Super)
+
+The 8GB card is the bottleneck. F5 + Kokoro can co-reside (~5GB +
+~1GB). Tortoise pushes the budget over and needs the GPU largely
+to itself — the `engine/tortoise` branch will carry the script
+that stops kokoro + f5 before a tortoise run and restarts them
+after. Replace with proper coordination once we have more VRAM.