fix(desktop): supervise and re-arm relay-mesh runtime (#2823)

Refs #2062

This carries forward the relay-mesh recovery work from #2304 by @Bartok9
(cherry-picked with original authorship/sign-offs) and adds the
startup/readiness supervision and packaging fixes found while validating
it against a real two-machine Buzz setup.

## From #2304

- Watch the local OpenAI ingress (`:9337`) after launch and re-arm a
stale relay-mesh runtime.
- Require consecutive failed probes before eviction so a transient
inference stall does not cause a cold restart.
- Bound stale-runtime shutdown, preserve runtime identity across the
asynchronous probe, and never evict a concurrent replacement.
- Re-arm only for agents that are actually running; deliberately stopped
agents stay stopped.
- Persist an actionable sentinel error under the managed-agent store
lock and clear only that error after recovery.
- Treat serve-to-client fallback as an intentional fail-safe; configured
serve restoration remains on its existing path.

## Added here

- Use the inference ingress (`:9337`), rather than management port
`:3131`, as Buzz's client-readiness boundary. A usable client no longer
fails or holds agent-save open merely because management startup is
still pending.
- Supervise the embedded SDK startup asynchronously, publish a pending
status while management is unavailable, and keep its mesh identity
alive.
- Avoid racing a replacement while SDK startup still owns the embedded
runtime. If that pending startup later loses ingress while a running
agent still needs it, request a controlled Buzz restart to reclaim the
otherwise-unreachable SDK thread.
- Defer roster-driven replacement while client management startup is
pending.
- Keep post-launch recovery in a dedicated module so the mesh entry
point remains within the desktop file-size gate.
- Explicitly mark generated Unix sidecars executable. On macOS, copying
over an existing non-executable destination preserved its old mode,
causing packaged `buzz-acp`, `buzz-agent`, and tool sidecars to be
reported as missing.

## Validation

Automated:

- `just ci` — passed, including formatting, Clippy with warnings denied,
desktop/web/mobile checks and tests, and builds.
- Full Tauri mesh-feature suite — 1,702 passed, 0 failed, 15 ignored.
- Mesh-feature Clippy with `-D warnings` — passed.
- Release macOS app bundle with `mesh-llm` — built successfully; every
bundled sidecar passed executable-mode and deep code-signature
verification.

Live two-machine E2E:

- M5: released Buzz serving `unsloth/gemma-4-26B-A4B-it-GGUF:UD-Q4_K_M`.
- Mac mini: this branch's packaged Buzz running a saved
`buzz-agent`/relay-mesh agent.
- Confirmed `:9337` accepted inference and the saved ACP harness started
with no error while `:3131` was still unavailable.
- Exact inference succeeded before restart (`CORRECTED-MINI-E2E-OK`).
- Bespoke Buzz shut down cleanly in 2.3s, then restored ingress and the
saved harness in 18.8s while `:3131` was still unavailable.
- Exact inference succeeded after restart (`AFTER-RESTART-E2E-OK`).
- A real Buzz `@C55` message traversed desktop → `buzz-acp` →
`buzz-agent` → mini `:9337` → M5 compute and published the requested
reply successfully.

---------

Signed-off-by: Bartok9 <danielrpike9@gmail.com>
Signed-off-by: Michael Neale <michael.neale@gmail.com>
Co-authored-by: Bartok9 <danielrpike9@gmail.com>
Co-authored-by: Michael Neale <michael.neale@gmail.com>

scripts/bundle-sidecars.sh

@@ -35,6 +35,14 @@ fi
 
 mkdir -p "$BINARIES_DIR"
 for bin in "${SIDECARS[@]}"; do
-    cp "$SRC_DIR/${bin}${EXE}" "$BINARIES_DIR/${bin}-${TARGET}${EXE}"
+    destination="$BINARIES_DIR/${bin}-${TARGET}${EXE}"
+    cp "$SRC_DIR/${bin}${EXE}" "$destination"
+
+    # cp preserves the mode of an existing destination on macOS. Generated
+    # sidecar placeholders may not be executable, so make the bundled Unix
+    # binaries executable explicitly.
+    if [[ -z "$EXE" ]]; then
+        chmod 755 "$destination"
+    fi
 done
 echo "Sidecars bundled for $TARGET"