feat(webview): checkpointRestoreLatestFile message type and handler (B3b v2-15a, epic #1375)

[easonLiangWorldedtech]

What

B3b v2-15a (epic Zoo-Code-Org#1375): the checkpointRestoreLatestFile webview→extension message — the per-file restore-latest contract and handler. Stacked on feat/fws-v2-14-change-card-l5 (base b1110b454). Split from the 492-line v2-15 source increment (7a1d4e61b4c1, upstream PR Zoo-Code-Org#1412) because it exceeds the 400-line soft budget; this PR is the API half (+167/−6, 173 shortstat lines).

• packages/types/src/vscode-extension-host.ts (+27/−5): CheckpointRollbackResult gains kind?: "rollback" | "restore-latest" (per-file results route to the control that requested them) and noOp?: boolean (a restore-latest that found no recorded write), plus a doc rewrite of the checkpointRollbackResult extension→webview message; checkpointRestoreLatestFile added to the WebviewMessage union with checkpointRestoreLatestFilePayloadSchema ({ cardTs: z.number(), filePath: z.string() }).
• src/core/webview/webviewMessageHandler.ts (+44): the checkpointRestoreLatestFile case — safeParse the payload, resolve the current task, lazily import("../checkpoints/rollback") and call restoreLatestFile(task, filePath), then post a checkpointRollbackResult with kind: "restore-latest", the noOp flag, and a conditional error. With no active task it posts success: false and error: "No active task to restore from." so the card can leave its pending state.
• src/core/webview/__tests__/webviewMessageHandler.rollback.spec.ts (+96/−1): a new describe("checkpointRestoreLatestFile") block with 4 tests — success outcome, the no-op flag surfaces, the error outcome, and the correlated no-task failure.

Base-parity decisions

The restoreLatestFile service already exists locally since v2-12 (PR #55), so this PR adds only the message contract, handler, and tests.

• webviewMessageHandler.ts and webviewMessageHandler.rollback.spec.ts byte-identical to upstream v2-15 (7a1d4e61b4c1).
• vscode-extension-host.ts: exactly one 1-line delta vs upstream — the documented v2-9 divergence: local keeps changeCardDetail?: ChangeCardDetail optional (upstream made it required). Upstream v2-15's kind/noOp additions land on top of that line unchanged.

Stryker (local diff gate, base b1110b454)

43 changed-code lines → 23 valid mutants: 23 Killed, 0 Survived, 0 NoCoverage, 0 timeout (extension lane; head 5ec15d56f). First run green — no new directives.

Gates

• pnpm check-types 11/11
• vitest on the rollback spec: 11/11 (4 new tests)
• eslint 0 (src package --prune-suppressions --max-warnings=0 and the packages/types base invocation), prettier clean
• diff budget: 3 files, +167/−6 (173 shortstat lines) < 400 soft cap — API half of the v2-15 split (the UI half follows as a stacked PR at 397 lines)

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