Drop the Python sidecar from the payload

[veronica-agent]

Why

The mouth has been the native qwen3-tts-worker (C++/GGML/Metal) since the native-tts PRs. Nothing calls the Python path any more, but the shipped payload still carried it: sidecar/say.py, pyproject.toml and uv.lock were embedded into the binary and extracted to ~/.cans/shipped, ship.Complete() refused a payload without them, and the Homebrew cask still declared uv as a dependency.

This removes all of it. Campaign workitem WI-8b1c5d.

What changed

• Deleted sidecar/say.py, pyproject.toml, uv.lock from the checkout and from internal/ship/fs/ (−2,000 lines, mostly the lock file).
• ship.Complete() now requires exactly voices/veronica/ref.wav + character.toml.
• Removed dead code in internal/ship: Sync, LookUV, ImportMLX, VenvReady, Venv, HFHome, Env, Sidecar (sync.go is gone). No callers outside tests.
• Tests
  • checkoutRoot() used sidecar/say.py as its checkout sentinel; without the fix TestEmbedMatchesCheckout would have started skipping instead of running. Sentinel is now go.mod + character.toml.
  • New TestEmbedIsExactlyPayload walks the embed FS and fails if it holds anything beyond the three payload files — the sidecar can't quietly come back.
  • New TestCompleteNeedsEveryPiece pins the Complete() contract.
  • TestPrepareSkipsSyncWhenSayBinSet → TestPrepareSkipsEnsureWhenSayBinSet, asserting what Prepare actually skips now.
• just dist stamp copies only the payload. Cask drops formula: uv. goreleaser check passes.
• just vhs demo baked the tape line with uv run … say.py. It now uses qwen3-tts-cli, the one-shot tool that ships next to the worker in ~/.cans/native/bin (-m models -t text -r ref.wav -o out.wav), and feeds the CLI's Total: N ms into the tape's ttfa field.
• Wording: no more MLX / sidecar in comments, error strings, or tape scripts. demo-say no longer shells out to python3 for a fractional sleep.

Verification

• go build ./... && go vet ./... && gofmt -l . clean; just test unit green, 8 packages.
• go test -run 'TestEmbed|TestComplete' -v ./internal/ship — all four run and pass (none skip).
• grep -rn 'uv run|mlx|say\.py|pyproject' over the tree: empty.
• Fresh CANS_HOME with the binary copied outside the checkout: cans doctor all ok, shipped/ holds only .stamp, character.toml, voices/veronica/{ref.wav,meta.json} — zero .py / pyproject / uv.lock.
• cans say "Put the cans on." speaks through the native worker (exit 0).
• just dist snapshot builds; archive is LICENSE, README.md, cans; strings on the binary finds no mlx_audio, sidecar/say.py, pyproject.toml, or UV_PROJECT_ENVIRONMENT.
• just vhs demo runs end to end without uv: 1.58 s line at 24 kHz mono, gif + mp4 regenerated and the booth frame shows the line with its ttfa.

Not in this PR

• docs/booth.gif / docs/booth.mp4 were regenerated as part of verification but reverted — the committed media is unchanged. Re-cutting the README demo with the native mouth is a separate call.
• Older installs may still have ~/.cans/venv and ~/.cans/hf from the Python era; nothing cleans those up yet (just uninstall --all wipes the whole home).
• -o, stdin, --stream, the mouth lock — that's the next slice (WI-a2e393), now unblocked.