feat(mcp): add a safe Unix stdio reload tool

[neusse]

Reconstructs #176 on current main, preserving Jean-Yves POCHEZ's original authorship while implementing the safety rework requested in review.

What changed

• Keeps the existing seven meta-tools as the cross-platform baseline.
• Advertises and dispatches reload_server only from the standalone Unix executable when it runs exclusively over stdio.
• Does not expose the tool on Windows, HTTP, mixed HTTP+stdio, or embedded/KiCad-hosted sessions.
• Preserves every original argument after argv[0], including an explicit --config path.
• Opens the installed executable once and proves its version before queuing the handoff.
• On Linux/Android, probes and executes the already-open file through /proc/self/fd, so a concurrent pathname replacement cannot change the selected executable.
• On macOS and other Unix targets without a portable fexecve, records device, inode, length, and modification time and revalidates both the open file and named path immediately before exec; ordinary atomic build/install replacement is refused.
• Refuses downgrades and requires allow_same_version=true for an intentional same-version development rebuild.
• Queues a one-shot reload plan instead of starting a timer inside the handler.
• Flushes the response and synchronous notifications, stops accepting requests, releases the run-registry record, and only then performs the one-way exec.
• Keeps the irreversible exec function in the standalone binary. The embedded library explicitly refuses any impossible reload handoff as defense in depth.

Why the transport handoff matters

The original 250 ms timer allowed the stdio loop to accept a second mutation before exec, potentially interrupting a write before cleanup guards ran. The transport now returns a sealed handoff before reading another request. The standalone entry point drops its run-record guard before exec, so the replacement process can register the same PID cleanly.

Evidence

Windows-local evidence:

• Focused reload tests cover capability gating, one-shot handoff, disabled dispatch, upgrade/equal/downgrade policy, and exact argument preservation.
• Negative control: temporarily neutralizing the downgrade refusal makes the version-policy regression test fail; restoring the guard makes it pass.
• Full gate passed:
  • cargo fmt --all -- --check
  • cargo clippy --workspace --locked --all-targets -- -D warnings
  • cargo test --workspace --locked --lib --tests
  • cargo test --workspace --locked --doc

Hosted Unix tests exercise the platform-specific handoff guards:

• Linux copies and opens the test executable, removes its pathname, and proves the open executable can still be invoked through its descriptor.
• macOS captures the candidate identity, proves an unchanged candidate is accepted, atomically replaces the named candidate, and proves the final handoff is refused.

Risk and rollback

The tool is absent unless the process proves the supported capability combination. The irreversible action requires confirm=true; same-version builds require a second explicit opt-in. The PR contains the preserved contributor commit plus one focused safety/adaptation commit, either of which can be reverted normally.

Supersedes #176.

[neusse]

Please reconstruct this PR from current main with only the unique Unix stdio reload change, then rerun all ten checks.

#442 has now merged, so the dependency is resolved, but this head is DIRTY against the resulting platform/startup code. The least-resistance path is a clean reconstruction rather than resolving the old cumulative history in place:

1. start from current upstream/main (296641b at this review);
2. carry only commit e106549a's unique reload behavior, adapting it to the merged run registry;
3. regenerate tool counts with cargo xtask fix-doc-counts if the tool remains in the final design;
4. verify the unique commit list and diff, then run the complete gate;
5. push the reconstructed exact head for review.

This is now status:waiting-on-author; it can re-enter the queue after that focused head is clean and green.