feat(nokv): add durable-completion read-side comparison probes

[NIU-123370]

Summary

Follow-up to the probe gap noted in #3195: examples/nokv-shadow-provider/probes.py covered only claim/CAS. This adds the read-side comparison the RFC's later runtime qualification slice registers — durable completion continuation projection (successor | no_followup | active_goal with fail-closed contradiction/dangling rules), reproduced by the provider with identical semantics.

Two probes:

• contract.durable_completion_projection — a bootstrapped open head is hand-evolved (the byte CAS stays the only writer) into the post-completion shape the future atomic complete_todo_with_successor will commit, read back through the provider, and projected with the LoopX durable_completion seam. All three typed continuations project correctly from the provider bytes, and re-reading + re-projecting yields identical outcomes (replay-stable).
• contract.durable_completion_fail_closed — a contradictory record (no_followup + successors) and a dangling declared successor both fail closed when read back through the provider.

Scope guardrails, per the claim confirmation in #3195: read-side projection comparison only — no provider completion command, no write-side atomicity. The module docstring and example README both state what these probes do and do not qualify.

Verification

• python3 examples/nokv-shadow-provider/probes.py contract from the repository root — all 8 probes pass (6 existing + 2 new), summary reports probes: 8.
• README's must-prove list and result-tag roster updated (six → eight).

Refs #3195 (probe gap named in review), RFC shared-goal-authority-state-provider-v0 later runtime qualification slice.

[huangruiteng]

评审结论：通过（Approve）

符合 #3195 指出的 probe 缺口：#3212 为 NoKV shadow provider 增加 durable-completion 读侧对比 probe——通过真实 seam project_durable_completion_outcome 把 provider 字节 CAS 写成的 post-completion head 投影为 successor | no_followup | active_goal，并覆盖矛盾（no_followup + successor 并存）与 dangling successor 的 fail-closed 行为；probe 注册与 README 说明同步更新。纯 examples 增量，未触碰运行时实现。

验证（exact head 792c3a473，独立 worktree）：

• python examples/nokv-shadow-provider/probes.py contract：8/8 probes 通过（含新增 durable_completion_projection 与 durable_completion_fail_closed）
• git diff --check：无输出；probe 模块语法检查通过
• GitHub Actions：pytest 正在运行（评审时 pending）

无阻塞问题。CI 转绿后即可合并。