…#4524)

## Summary

Official Linux desktop packages (`.deb` / AppImage) are built without
`--features mesh-llm`, so they ship the `mesh_llm_stubs` backend and
Settings → Compute always fails with `mesh-llm feature not enabled`.
This PR adds the feature flag to the two Linux build commands:

- `release.yml` → `release-linux` job
- `linux-canary.yml` → canary build

That's the whole diff — 2 lines. Fixes #3788 (Linux); see also #3841
(dup with UI-gating PR #3914) and the Windows twin #2836/#3223.

## Why no native prebuild step (unlike the macOS job)

The macOS job carries Metal llama prebuild/cache steps from #798. Linux
doesn't need an equivalent:

- `mesh-llm-host-runtime` is compiled with `dynamic-native-runtime` and
installs the recommended runtime on first use (verified by sha256
checksum over HTTPS; upstream's signature verification path is not yet
implemented — default policy is `RequireChecksum`, per
`mesh-llm-runtime-install/src/lib.rs`)
(`desktop/src-tauri/src/mesh_llm/mod.rs` —
`initialize_mesh_native_runtime`), so release builds work on clean
machines without bundling llama.cpp.
- Upstream publishes Linux x86_64/aarch64 runtime bundles for the pinned
`v0.74.0` line, and `scripts/ensure-mesh-native-runtime.sh` already maps
`meshllm-native-runtime-linux-x86_64-cpu` / `linux-aarch64-cpu` for
local/e2e use.
- The unmerged branch `micn/mesh-node-download` (`96f29417a`) treats
even the macOS prebuild steps as removable dead weight for the same
reason.

## Background

The omission is historical drift, not a decision: Linux packaging
predates the mesh feature flag (#693), mesh became opt-in for
build-cost/reliability reasons (#823, #1183), and #1221 re-enabled it
for releases by editing only the macOS build line. `release-linux` and
the later `linux-canary` copy were never revisited.

The mesh shutdown hard-exit/relaunch path is gated `all(mesh-llm,
target_os = "macos")` because ggml/Metal destructors abort on macOS;
ordinary mesh shutdown (`shutdown_mesh_runtime`) is cross-platform, so
Linux falls through to the generic path.

## Validation

- [x] `./bin/cargo check --manifest-path desktop/src-tauri/Cargo.toml
--features mesh-llm` green at base `2c0ac2467` (feature graph compiles
at the pinned v0.74.0 line)
- [ ] Linux canary run with this change: AppImage/.deb build succeeds
and binary contains real `mesh_llm` symbols (not `mesh_llm_stubs`)
- [ ] Installed package: cold-start → Settings → Compute → runtime
download → serve → clean shutdown

The last two need a Linux run/host. **Note (from review):**
`linux-canary.yml` is `workflow_dispatch`-only and its `Require main`
step rejects non-main refs, so the canary cannot run on this branch
pre-merge — and `.github/workflows/**` matches no ci.yml paths-filter,
so this PR's own CI does not exercise the changed lines. Validation
sequencing is therefore merge → dispatch linux-canary on main →
live-package pass, with a trivial 2-line revert as the escape hatch.

Signed-off-by: npub1qyvc0c5kl4gqv2fd97fsk46tu378sqgy35vc83rvgfwne90sel7s0ed67d <011987e296fd5006292d2f930b574be47c7801048d1983c46c425d3c95f0cffd@buzz.block.builderlab.xyz>
Co-authored-by: npub1qyvc0c5kl4gqv2fd97fsk46tu378sqgy35vc83rvgfwne90sel7s0ed67d <011987e296fd5006292d2f930b574be47c7801048d1983c46c425d3c95f0cffd@buzz.block.builderlab.xyz>