Centralize terminal lifecycle mutations

[vorporeal]

Description

Stack PR 4/8; depends on #12854.

Context

Normal command completion is not a single block mutation. It also cleans up terminal-wide state, finalizes background and in-band bookkeeping, advances the block list, emits ordered shared-session state, and notifies downstream handlers. Prompt and pre-execution hooks have similarly coupled side effects spread across TerminalModel, BlockList, and Block.

Adding recovery directly to those existing paths would require duplicating that behavior for every exceptional sequence. That would make it easy for recovered commands to miss cleanup, emit events in a different order, or complete the same block twice.

Approach

This PR is a behavior-preserving refactor that establishes one normal lifecycle mutation pipeline before any coordinator or recovery policy is added. BlockList and Block gain focused primitives for classifying next-block IDs, ensuring a block is started, completing and advancing the active block, applying a prompt, and applying pre-execution evidence. TerminalModel becomes the single place that composes those primitives with model-wide completion, prompt, and handler side effects.

Normal CommandFinished, Precmd, and Preexec hooks are routed through the new helpers, but exceptional sequences are not recovered yet. Keeping this slice policy-free makes normal-flow parity independently reviewable before later PRs reuse the same pipeline for recovery.

Review guidance

• Does the centralized path preserve the previous ordering and exactly-once behavior of cleanup, block events, ordered shared-session events, and handler notifications?
• Are the new block-level helpers mutation primitives rather than places that independently decide lifecycle policy?
• Does this PR avoid changing how exceptional or out-of-order hooks behave?

Testing

• Added normal lifecycle parity coverage for completion and prompt side effects, bootstrap, user, in-band, background-finalization, alternate-screen, and shared ordered-completion paths.
• Final-stack validation: cargo check -p warp --tests, ./script/format, and git diff --check.

[vorporeal]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Enable terminal lifecycle recovery in dogfood #12859
• Recover missing CommandFinished hooks from Precmd completion metadata #12858
• Safely reconcile repeated and prompt-only Precmd hooks #12857
• Gate terminal lifecycle transitions with a coordinator #12856
• Centralize terminal lifecycle mutations #12855 👈 (View in Graphite)
• Distinguish Precmd hooks with completion metadata from prompt-only hooks #12854
• Add command completion metadata to Precmd #12853
• Document terminal block lifecycle hardening design #12852
• master

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