CLI: --models-dir flag + default to ~/.local/share/Vernacula/models

[christopherthompson81]

Motivation

Vernacula.CLI currently overloads `--model

` as both the Parakeet
bundle path and the models root for sibling backends. That's a
historical artefact — Granite, Cohere, Qwen3, VibeVoice, and Whisper all
do `Path.Combine(modelDir, Config.SubDir)` and so silently treat
`--model` as the root. But Parakeet (`new ParakeetAsr(modelDir, ...)`)
and `VadSegmenter` load files flat from the directory you pass; they
need a bundle, not a root.

Practical consequence: pointing `--model` at the desktop app's
`~/.local/share/Vernacula/models` works for Granite/Cohere/Qwen3/VibeVoice/Whisper
but breaks Parakeet and VAD because they look for files at the root
instead of in the per-backend subdirs.

The desktop app sidesteps this with `SettingsService.GetModelsDir()`
(root) plus per-backend `GetModelsDir()` (root + canonical subdir).
The CLI should match.

Scope

1. Add `--models-dir ` as the canonical "where everything lives"
   flag, mirroring the desktop app's models root.
2. Make `--model` and `--models-dir` both optional. When neither is
   passed, default to `DefaultModelsDir()` — which already exists in
   Program.cs (line 957) and resolves to
   `~/.local/share/Vernacula/models` on Linux. Today the CLI errors with
   "--audio and --model are required"; that becomes "--audio is required"
   and the models root falls back to the desktop app's location.
3. Add subdir-fallback at flat-load sites. Same pattern Sortformer
   already uses via `Config.GetSortformerModelPath`: when given a root
   that contains a per-backend subdir, prefer the subdir; else fall back
   to root for back-compat with users passing a bundle directly. Sites
   to update:
   • Parakeet construction (Program.cs around line 780):
     `Directory.Exists(Path.Combine(modelDir, Config.ParakeetSubDir)) ? ... : modelDir`
   • VadSegmenter (line 301): same treatment with `Config.VadSubDir`
     ("silero").
   • DiariZen (line 312–313): already does this; verify it covers the
     desktop app's layout.
4. Update `PrintUsage()` to document the new flag, the default, and
   the convention.

Out of scope

• Rewriting the per-backend `--cohere-model` / `--qwen3asr-model` /
  `--vibevoice-model` / `--granite-model` overrides. Those stay as
  per-backend escape hatches when someone wants to point at a non-canonical
  bundle location.
• Changing `Vernacula.Base` constructor signatures (Parakeet, VadSegmenter)
  to do their own subdir resolution. Keep the resolution in CLI; Base stays
  bundle-path-agnostic. Sortformer's `Config.GetSortformerModelPath` is the
  one already-existing exception; leaving it is fine.

Definition of done

• `dotnet run --project src/Vernacula.CLI -p:EP=Cuda -- --audio foo.wav --asr granite`
  with no `--model` / `--models-dir` succeeds, picks up the desktop app's
  `~/.local/share/Vernacula/models` automatically, and runs Granite.
• The same invocation with `--asr parakeet` (no `--model`) succeeds and
  resolves Parakeet from `/parakeet/`.
• `--model ` (the existing pattern) still works
  unchanged for users who have a non-canonical layout.
• `--help` documents the flag + default.

Context

• Surfaced while running Granite Speech: another perf round — GPU under-utilization signal #41 measurements — typing
  `--granite-model ~/.local/share/Vernacula/models/granite_speech_4_1_2b_bf16`
  on every command got old fast.
• The default helper `DefaultModelsDir()` already exists in
  `src/Vernacula.CLI/Program.cs:957`.