Prevent self-owned selection deadlock after paste

[c-wri]

Why

Repeated operations could leave the daemon alive but permanently non-responsive until restart, violating the bounded command-ready invariant from docs/architecture.md.

The escaped failure class was identified: after ClipReg restores a selection, ClipReg may itself be the current clipboard source. snapshot_current() already protects this boundary by cloning the held MIME object, because synchronously receiving ClipReg's own Wayland offer would require source_send() on the same single-threaded event loop. stable_snapshot_current() bypassed that boundary and called the raw offer-receive path directly, allowing a later paste to deadlock while snapshotting the restored clipboard.

What

• Added a direct runtime lifecycle regression for a compositor-visible self-owned clipboard offer.
• First checkpoint intentionally proved the escaped behavior on the current beta and in Ubuntu CI.
• Corrective checkpoint routes stable snapshots through the existing snapshot_current() self-ownership boundary.
• Added phase-only grab/paste diagnostics with operation ID, phase, status, and duration; clipboard payloads are never logged.
• Wired the lifecycle regression into strict and sanitizer gates.

No new daemon, thread, watchdog, timing workaround, public command, config surface, or version bump is introduced.

Validation

Negative proof before the correction:

• PR checkpoint e10179ee3ee1167eb7ef62a434247466da33faac failed make check in Ubuntu 24.04 CI at [8/12] runtime lifecycle regressions with stable snapshot rejected a self-owned selection.

Corrective automated proof:

• Local canonical make check passed on the supported workstation checkout, including offline regressions, ASan/UBSan, real wayland-scanner, installed libwayland-client, and binary self-test.
• Corrective checkpoint e445ee5e99d216ee2c9d42d3352eac049905cb46 passed GitHub CI / Check on Ubuntu 24.04.
• CI also passed the tracked-state-clean verification.

Live COSMIC proof:

• Pop!_OS 24.04 / COSMIC Terminal (com.system76.CosmicTerm).
• 39 traced grab/paste operations in the captured journal window.
• Repeated operations exercised snapshot-clipboard-clone-owned and snapshot-primary-clone-owned after restored selections.
• Multiple terminal-grab failures returned bounded ETIMEDOUT results through transaction-finish; later operations remained command-ready.
• NRestarts remained 0 -> 0.
• Post-loop clipreg list, clipreg doctor, and clipreg app all responded successfully.
• The prior alive-but-permanently-unusable state did not recur.

Release impact

The liveness blocker in issue #1 is satisfied by this candidate and may close when this PR is integrated. No prerelease is cut for this PR.

Two separate terminal behavior defects exposed by the same qualification remain blockers for v0.2.0-rc.1: #5 (generic Copy probe can leak terminal escape sequences) and #6 (clipboard-only paste unnecessarily disturbs primary-selection affinity).

Open / deferred

• Terminal grab must not leak synthetic key escape sequences into the shell #5 and Clipboard paste must preserve unrelated terminal primary-selection affinity #6 are separate runtime review units and are intentionally not folded into this liveness PR.
• Build-warning cleanups Remove Make target cycle between build command and build directory #3 and Make bundled COSMIC toplevel protocol subset DTD-valid #4 remain separate foundation/tooling issues.