menu bar: switch models, and keep only the one in use on disk - #21

WhisperKit's downloadBase defaults to ~/Documents/huggingface. With iCloud
Desktop & Documents sync on, the model weights are replicated and, once the
disk fills, evicted to dataless placeholders (SF_DATALESS, st_blocks=0).

CoreML mmap()s the weight blob during ANE compilation, and mmap of an evicted
file blocks indefinitely: the daemon prints "loading <model>..." and never
reaches "listening on fn hold". A partially materialised read instead fails as
CoreML error 3, "Failed to read first word from AudioEncoder.mlmodelc/
coremldata.bin. It is not a valid .mlmodelc file." Observed on macOS 26.4.1
with whisper-large-v3-turbo (1.6 GB) on a 94%-full disk.

Point downloadBase at Application Support, which is neither user-visible nor
sync-managed. Existing installs keep their Documents copy; note it on load so
the space is reclaimable rather than silently abandoned.

Adds Parakeet TDT v3 alongside WhisperKit. Measured on the same eight
recordings: an order of magnitude faster (0.07-0.11 s against 0.6-2.5 s
per utterance) and better on short utterances carrying English technical
terms, worse on long sentences. Which side of that trade matters depends
on how a person dictates, so it is a choice in the menu rather than a new
default.

The model is picked from a Model submenu and remembered like the input
device. `--model` still wins for the launch that passes it. Switching
loads the incoming model before dropping the current one, so a failed
download leaves the user dictating with what they had, and the preference
is written only after the load succeeds.

Whatever is not in use is deleted: 792 MB reclaimed switching away from
large-v3-turbo-compressed, plus 11 MB of tokenizers that live outside the
weight directory. The same sweep runs at startup, so models stranded by a
crash or an older version come back as free space.

whisperKitID becomes engineID: the field now holds Parakeet ids too.

Picking a model left the menu unchanged until the load finished, which for
a model that is not on disk yet means minutes. The row you clicked kept
the old tick, the state line kept the old name, and the only honest
reading was that the click had been ignored.

The row now takes a dash the moment it is clicked, the other rows go
quiet until the switch settles, and the state line carries the download
as a percentage.

Both engines report the download and neither reports the load, so the
line drops the number and says loading for the last stretch rather than
sitting at 100%. WhisperKit only reports progress on its static download,
so the weights are fetched first and the pipeline is pointed at the
folder afterwards.