feat(tts): make Silero Native the default engine (aidar, 24000 Hz)

[xilec]

What

• Fresh installs and configs missing the keys now default to the Silero
  Native engine with speaker aidar at 24000 Hz (was Piper / xenia /
  48000).
• Existing config.json files are untouched: serde defaults only fill
  absent keys, and a missing model bundle still falls back to Piper for
  the run with the config value preserved.
• Settings: saving while the engine selector is coerced to the fallback
  (saved engine unavailable) no longer persists that fallback — engine
  is omitted from the patch (buildSettingsPatch), so a bundle-less
  fresh install keeps the silero_native default until the bundle is
  downloaded.
• Engine option labels cleaned up: the native engine is listed first as
  «рекомендуемый», «Python ttsd» → «Python», Piper's «без Python» note
  dropped.
• Docs (README ×2, install, index, conventions, architecture) and
  CHANGELOG updated to the new engine roles.

Specs

OpenSpec change archived in
openspec/changes/archive/2026-08-03-silero-native-default-engine/ —
delta specs synced into storage (config defaults), ipc-commands
(UIConfig annotations + fresh-install scenario), and ui (Settings
coerce-save scenario).

Verification

• just test — Rust 974, silero-native 68 + bundle-gated suites,
  TS 131, Python 65: all green.
• just lint and openspec validate --specs --strict green.
• Manual: app launched with a keyless config starts on Silero Native /
  aidar / 24000; an existing user config is unaffected.