[split-1066] U4 - feat(write-to-file): per-task partial stream state + cleanup primitives #1934

Unit U4 of the PR #1066 split (2/6).

Split plan: #703
PR: #1929
Content source of record: tag pr1066-source = 46d1d218701f0ce2d675b1b315489bacb6b0f77d (easonLiangWorldedtech/Zoo-Code).

Unit contract

Per-task partial stream state keyed by taskId.instanceId, the TaskAborted teardown listener registered once per task, clearTaskState(), per-task path stabilization, the cleanup primitives (resetDiffViewAfterWrite, revertDiffChangesBeforeReset, finalizePartialToolAskAfterFailure), and the ClineProvider disposal wiring that releases the state for a task disposed without an abort.

Why this unit exists on its own

Single provider group (WriteToFileTool streaming state + provider wiring) + single gate scope. Everything downstream depends on this unit's state keying, so it must land first.

Boundary

• base: cf5abe64d6e8 (previous unit head, U12)
• head: 52699c6cdc7d
• chain: U12 -> U4 -> U5
• merge order is fixed: U12 -> U4 -> U5 -> U3 -> U6 -> U7/FINAL; the FINAL integration PR (fix(write-to-file): clean partial state on missing-param and rooignore denial + integrate the #1066 split series (6/6 + FINAL) #1928) is the sole merge target.

Files and budget

• src/core/tools/WriteToFileTool.ts +163/-2 — content subset of source
• src/core/tools/__tests__/writeToFileTool.spec.ts +116/-0 — byte-identical to source
• src/core/tools/__tests__/writeToFileTool-partial-state-cleanup.spec.ts +100/-0 — allowNew (sanctioned new content)
• src/__tests__/removeClineFromStack-delegation.spec.ts +41/-0 — byte-identical to source
• src/core/webview/ClineProvider.ts +6/-0 — byte-identical to source
• budget: 428 a+d / 5 files — SOFT-OVERSHOOT (rationale: the focused cleanup spec is new content; under the 1000 hard cap)
• mutation gate: 63 changed executable lines (58 WriteToFileTool + 5 ClineProvider) — under the 500 cap; valid mutants for the whole PR are 116 / 400, so no directive was added by this unit.

Verification (must pass by once, binary)

• zdt split verify --contract U4.json --worktree <wt> --head 52699c6cdc7d — PASS (every changed file is a content subset of the source of record, or an explicitly sanctioned allowNew file).
• Tests: 230 passed / 5 skipped, exit 0 (narrowest relevant suites: Task.spec.ts, writeToFileTool.spec.ts, writeToFileTool-partial-state-cleanup.spec.ts, removeClineFromStack-delegation.spec.ts, presentAssistantMessage-custom-tool.spec.ts).
• Changed-line coverage: 34 covered / 0 uncovered — PASS.
• ESLint --prune-suppressions --max-warnings=0: clean on every touched file; suppression counts unchanged.
• No .changeset file, no CHANGELOG edit (AGENTS.md).

Deviations recorded

• Accepted mechanism divergence: sibling streaming tools (ApplyDiffTool, EditFileTool, SearchReplaceTool, EditTool) still use BaseTool's singleton lastSeenPartialPath/resetPartialState. Lifting the per-task keying to BaseTool is a follow-up PR, deliberately not done here.
• New focused spec added so the primitives' catch arms are proven at this layer: they are only reachable by calling the primitives directly, because the tests that trigger them through execute()/onParameterParseFailure() belong to U5/U6.

Reproduce

git fetch https://github.com/easonLiangWorldedtech/Zoo-Code p1066/u4-per-task-stream-state
node zdt.mjs split verify --contract U4.json --worktree <wt> --head 52699c6cdc7d
node zdt.mjs split measure --worktree <wt> --base cf5abe64d6e8 --head 52699c6cdc7d
pnpm --dir src exec eslint --prune-suppressions --max-warnings=0 <touched file>