fix: reclaim posix harness trees orphaned by a crashed meshloop

[smota]

Implements #30
Closes #30

Problem

On POSIX, a harness process group outlives an aborted Meshloop, because a group has no equivalent of the Job Object's kill-on-close. On restart, recovery saw the attempt as untracked, recorded HarnessCrashedOrTimeout and retried, while the old tree kept running against the worktree.

Change (option 1: recovery reconciliation, safe Rust)

• RunLoop::reattach_running: a Running attempt the harness no longer tracks is cancelled before the crash is recorded.
• CliHarness::cancel: for an untracked attempt with a recorded pid, calls the new process::kill_orphaned_tree.
• kill_orphaned_tree guards against pid reuse. It kills only when the pid is alive and runs the harness executable (image name on Windows; comm on POSIX, which Linux truncates to 15 bytes), and on POSIX only when the pid still leads its own process group.
• ADR 0025 amendment records the decision, the rejected alternatives and the residual risks. The verification note for grandchild_dies_on_parent_abort now says it is Windows-only.

Known limits

• Orphans are reclaimed at the next resume or status of that graph, not immediately.
• A harness launched through a shim whose process image differs from the configured executable is not matched, so it is left running rather than risk killing an unrelated process.
• There is no engine-level reattach test with fakes; the engine change is a single cancel call covered by the adapter tests and the end-to-end suites.

Merge note

#38 also appends an amendment to ADR 0025. Whichever merges second needs a trivial rebase; I will handle it.

Validation

• cancel_kills_an_orphaned_tree_left_by_a_previous_process fails without the untracked-pid branch ("orphaned leader 36968 survived cancel") and passes with it.
• cancel_leaves_an_unrelated_process_with_a_recorded_pid_alone covers the pid-reuse guard.
• Windows: xtask check (17 suites) and smoke pass.
• Linux (WSL2 Ubuntu 24.04, native clone at 3da9a42): xtask check (17 suites) and smoke pass, with 0 leftover fixture processes.

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