fix: bundle espeak-ng-data inside binary (fixes #59)

[pszymkowiak]

Summary

Piper TTS failed on fresh installs (vox setup → test piper backend) with No such file or directory, as reported in #59.

Root cause

The espeak-ng C library that piper-rs links statically has its data directory path baked in at compile time — the path of the CI runner that built the release binary. On end-user machines that path does not exist, so espeak_Initialize aborts with No such file or directory for every language.

Confirmed by strings on the released vox-aarch64-apple-darwin binary:

/Users/runner/work/vox/vox/target/aarch64-apple-darwin/release/build/espeak-rs-sys-<hash>/out/share/espeak-ng-data

espeak-rs does provide a fallback search path (env var PIPER_ESPEAKNG_DATA_DIRECTORY, then CWD, then exe dir), but the release tarball only contains the vox binary — no data files — so none of those resolve.

Fix

Embed the compiled espeak-ng-data into the binary and extract it on first piper use:

1. build.rs — locates espeak-ng-data in espeak-rs-sys's OUT_DIR (cargo guarantees dep build scripts have already run) and stages it into vox's OUT_DIR.
2. piper.rs — embeds the staged directory via include_dir!("$OUT_DIR/espeak-ng-data"), extracts it once to ~/.config/vox/piper/espeak-ng-data (with a sentinel file), then sets PIPER_ESPEAKNG_DATA_DIRECTORY before any Piper::new / model.create() call.

Binary size grows by ~15 MB (the size of espeak-ng-data). No network access required, no install-script changes needed — fresh installs just work.

Test plan

• CI builds green on macOS, Linux, Windows
• Manual: fresh install on M-series Mac, vox setup → test piper (en) → audio plays
• Manual: same, switch to French in TUI → audio plays
• Manual: second run is fast (sentinel file short-circuits re-extract)