[core] Expose rollout-history turn boundaries

[adamvy-oai]

Overview

This is the fourth change in a five-PR stack that improves reusable Rust APIs around the in-process app server and stored rollout history. It moves user-turn boundary classification out of context-manager internals so other history consumers can apply the same rule.

What

• add codex_core::rollout_history
• expose is_user_turn_boundary() for persisted response items
• move existing core callers to the neutral module
• cover user input, contextual fragments, agent messages, and assistant instructions with focused classification tests

Why

User-turn boundary classification describes persisted rollout history, but its implementation previously lived inside the context manager. Consumers reconstructing stored history should not depend on a context-manager implementation detail to apply the same classification.

Keeping the helper in codex-core also avoids moving contextual-fragment knowledge into codex-rollout, which would require a broader dependency change.

Stack

Each PR is based on the preceding branch:

1. #28459 — custom in-process thread stores
2. #28460 — lossless in-process event delivery
3. #28461 — stored-thread view helpers
4. #28462 — rollout-history turn boundaries (this PR)
5. #30369 — durable external thread goals

Validation

Validation for the final stacked commits:

• focused codex-core rollout-history tests
• just test -p codex-app-server: 886 passed; one timing retry passed on its second attempt
• just test -p codex-core: 2,717 passed; eight timing-sensitive failures passed in focused reruns, while five high-output code-mode failures remained
• just test --test-threads=8: 10,944 of 10,958 passed; the remaining failures were the same five high-output tests, three timing-sensitive tests that pass in focused reruns, and six tests in untouched exec/TUI code affected by this host's managed permission policy
• a representative high-output failure reproduces on the exact upstream/main commit
• just fix -p codex-app-server, just fix -p codex-core, and just fmt; final worktree clean

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