core: track thread publication lifecycle

[winston-openai]

Summary

• register every loaded thread under its stable ThreadId in a manager-scoped tree
• retain the parent relationship and a private incarnation generation so stale handles cannot mutate a replacement thread
• keep child initialization active until the thread is actually published, and reject registration beneath a closing ancestor
• prune closed thread trees leaf-to-root without time-based state

Why

The follow-up descendant-activity gate needs a durable identity tree keyed by the same thread IDs used by app-server and ChatGPT. This stage owns only registration, publication, replacement isolation, and cleanup; activity reservations and completion delivery remain in the next PR so each concurrency contract is independently reviewable.

Stack

• prerequisite: core: make idle thread shutdown atomic #31304 provides local atomic idle shutdown
• this PR: thread publication lifecycle (316 changed lines)
• follow-up: descendant activity and terminal-delivery gating with public integration coverage
• app-server config reuse remains in app-server: reuse loaded threads when resume config matches #31282 above the core layers

Validation

• CARGO_INCREMENTAL=0 just test -p codex-core registration_and_publication_preserve_thread_incarnations dropped_tree_handles_are_pruned_leaf_to_root
• CARGO_INCREMENTAL=0 just fix -p codex-core
• just fmt

The two focused tests passed before the final fix/format pass; tests were not rerun afterward, per repository guidance.

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[winston-openai]

@codex review

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