chore(release): v0.5.1 — v1 contracts frozen, 9–19× faster agent wire, rebirth-proof IPC

[NickFlach]

Release prep for v0.5.1 (VERSION bump + CHANGELOG). Tag v0.5.1 will be pushed after this merges (tag-triggered release per ADR-0018; tag must match VERSION).

Everything since v0.5.0 (2026-07-11):

• ADR-0020 v1 contract freeze COMPLETE — syscall ABI golden (fix(abi): ADR-0020 pre-freeze corrections to the guest syscall ABI header #206–docs(adr): ADR-0020 Accepted — the v1 guest syscall ABI is frozen (#206–#211) #212), COM2 wire/attestation golden + MCP tool-schema golden (feat(contracts): freeze MCP tool schemas + COM2 wire/attestation format (ADR-0020 complete) #226), all teeth-checked
• ADR-0021 honest memory Accepted (feat(mem): PMM 1GB-clamp + floor hardening, no-op at 128M (ADR-0021) #213–feat(mem): high-memory writeback proof + differential CI; ADR-0021 Accepted (PR-4) #216, fix(mem): clamp pmm_highmem_selftest rover to the first high frame (ADR-0021) #219)
• ADR-0022 prerequisites (feat(sched): scheduler perf/stability baseline recorder (ADR-0022 prereq 2, PR-1) #217–feat(sched): arm the scheduler preemption-rate gate; ADR-0022 Accepted (prereq 2, PR-2) #218) and the latency epic itself (feat(sched): the I/O-priority boost — COM2/IPC latency fix (ADR-0022 epic) #225): I/O-priority boost — PING ~9×, STATUS ~19× faster
• ADR-0023 rebirth-proof IPC (docs(adr): ADR-0023 Proposed — IPC peer re-wiring on watchdog rebirth + dead-target unlink #222–feat(ghost,ci): ghostd exit-is-a-feature + ADR-0023 Part-1 integration gate #224): no capability outlives the process it names; declarative re-mint; exit-is-a-feature for ghostd
• Fix sweep (fix(user): terminate recv_msg buffers in the echo/client demos (self-disclosure) #220 recv_msg self-disclosure, fix(ci): make the browser boot-gate interactivity probe flake-proof (use uptime) #221 browser-gate flake)

Note: this is version-numbered per the maintainer's call (v0.5.1); content-wise it would also justify v0.6.0 — flagging in case you want to bump before tagging.

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