[kitlangton]

Closes #35020.

Summary

Closes the remaining V2 event-audit cleanup after the fragment and execution/error lanes.

• Enforces tool settlement before the owning step terminal event across clean, provider-error, thrown-LLM-error, content-filter, hosted-tool, interruption, and defect paths.
• Emits exactly one session.step.ended or session.step.failed per step.
• Adds assistant-scoped exact-order regression tests for local and hosted tools.
• Documents the cross-drain orphan settlement exception and step-scoped callID uniqueness.
• Rewrites compaction checkpoint headings around continuation/resumption semantics.
• Removes generated legacy SDK duplicate names only after normalized shape-equivalence checks.
• Fails loudly if SessionNext-style generated noise reappears.

Checkpoint Vocabulary

## Continuation Goal
## Operating Constraints
## Progress
### Completed
### In Flight
### Blocked
## Decisions To Preserve
## Resume From Here
## Context To Preserve
## Working Files

Verification

• 170 runner-adjacent Core tests passed after rebasing onto feat(core): add execution lifecycle and structured errors #35280
• SDK typecheck and test passed
• Full-repo pre-push typecheck passed
• Legacy SDK generation reproduced identically on consecutive runs
• No Shell1V2, numbered SessionStructuredError, or SessionNext duplicate noise remains
• Prettier clean; file-scoped oxlint has 0 errors
• Independent settlement-order and SDK-safety reviews passed after coverage/equivalence hardening

No schema migration is required for this behavior/template/codegen cleanup.

Post-Merge

After the stack lands, close #35020 and #35021 with shipped/deferred mappings, then remove the temporary event-audit coordination plan in a final closeout change.

[kitlangton]

Collapsed into #35272 so the Session event contract lands in one PR with one V2 history reset.