M7.9: Text to Speech narrations (panel, tts.* commands, built-in voice)

Write what a narrator says, pick a voice, pitch and pace, and add it to
the timeline. Selecting a narration clip loads it back into the panel;
Save regenerates it in place as one undo step, keeping the clip's start
and length (longer speech kept whole in the file, shorter speech padded
with silence).

- filmcraft-tts (L2): Voice trait, voice catalogue, pause markers
  ([pause 1s]), and an original formant synthesizer (Basic Female/Male;
  robotic placeholder until the neural voices land).
- project: Narration + Project::narrations (schema v13, no-op migration).
- engine: tts.voices/preview/create/edit/inspect; narrations never
  overwrite clips (free targeted track, free track, else a new track).
- ui: Text to Speech panel beside Properties, preview playback,
  click-to-edit, automation ids.

Gates: fmt, clippy -D warnings, layers, assets and wasm (rustup
toolchain) pass. Tests: all pass except three that fail identically on
main (61238bf) on this machine: shortcuts_tests::assign_reassigns_
conflicts_and_undoes, gpu fx_tests::gpu_effects_match_cpu_exactly, and
the frame pool test, which passes when run alone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

xtask/src/main.rs

@@ -60,6 +60,7 @@ const LAYERS: &[(&str, u8)] = &[
     ("audio", 2),
     ("captions", 2),
     ("speech", 2),
+    ("tts", 2),
     ("interchange", 2),
     ("render", 3),
     ("gpu", 3),