fix(transcription): compile the GGUF session owner on Linux

[burak-s]

Summary

main does not compile for the Linux target. Check Linux on 61d401a fails in both jobs at the same error:

• test: cargo test --locked --bin voice-control
• nix: nix build .#checks.x86_64-linux.package, at Compiling voice-control v2.1.19

error[E0432]: unresolved import `crate::transcription`
   --> src/linux_transcriber.rs:8:12
    |
  8 | use crate::transcription::OfflineGgufSession;
    |            ^^^^^^^^^^^^^ could not find `transcription` in the crate root
    |
note: found an item that was configured out
   --> src/main.rs:123:5
    |
122 | #[cfg(target_os = "macos")]
    |       ------------------- the item is gated behind the `macos` feature
123 | mod transcription;

The previous main push, 983c71c (run 35392493576), fails the same way.

Reproduction: git checkout 61d401a && cargo check.

Cause

2324ee4 moved OfflineGgufSession into src/transcription.rs and pointed the Linux-only src/linux_transcriber.rs at it, but src/transcription.rs is #[cfg(target_os = "macos")] and cannot simply be ungated: it depends on the macOS-only apple_speech and parakeet modules.

The move itself was right. Before 2324ee4, Linux held a raw transcribe_cpp::Session and never had the bounded-scratch session handling. This keeps one shared owner and only moves it to where both platforms compile it.

Change

• add src/offline_gguf.rs: the OfflineGgufSession owner moved verbatim, gated for macOS and Linux;
• src/linux_transcriber.rs and src/parakeet.rs import it from the new module;
• src/transcription.rs drops its now-duplicate definition.

macOS behavior is unchanged: a move plus two import lines. .github/workflows has no macOS job, so that half relies on the release build as before.

Placement note: src/transcription.rs is the natural home but is macOS-only, and src/transcription_models.rs owns the catalog and installation rather than inference. Happy to relocate it if you prefer another module.

No map or doc changes: no user-facing behavior, entry point, default, prerequisite, or recovery path changes.

Verification

x86_64 Fedora 44, local, non-root:

cargo fmt --all --check                                            # pass
cargo build                                                        # pass
cargo test --locked --bin voice-control                            # 145 passed; 0 failed; 7 ignored
cargo clippy --locked --bin voice-control --tests -- -D warnings    # pass
cargo clippy --locked --all-targets --all-features -- -D warnings   # pass
git diff --check                                                   # pass

The 7 ignored tests are the Xvfb-gated X11 grab tests (linux_input::tests), which need the workflow's isolated display. Wayland paste, virtual-microphone dictation, service IPC, and installer checks are left to CI.

The Check Linux run for this PR is action_required pending maintainer approval (run 35442068703), so the results above are from a local run rather than CI on the branch.