payload: add-only stale marker for read-only stale menubar serves

[iamtoruk]

Summary

Closes #771. The warm-refresh gate (#743) serves the prior complete snapshot when the refresh lock times out or is unavailable — correct and fail-closed, but the payload's only time field was the render timestamp, so a stale serve was indistinguishable from a fresh parse.

Since #771 was filed, src/parser.ts gained isSessionHydrationComplete() (PR #937) which already returns false exactly when the last completed parse in-process was a read-only serve that had to skip or stale real on-disk changes. This PR threads that signal through to the payload:

• src/menubar-json.ts: new optional top-level stale?: boolean on MenubarPayload; buildMenubarPayload gains a trailing optional stale param and sets payload.stale = true only when passed true — never emitted as false.
• src/usage-aggregator.ts: buildMenubarPayloadForRange passes isSessionHydrationComplete() ? undefined : true.
• app/renderer/lib/types.ts: mirrors the field (add-only, per this repo's CLI-app payload contract).
• mac/Sources/CodeBurnMenubar/Data/MenubarPayload.swift: mirrors the field in the Codable data model (let stale: Bool?, decoded via decodeIfPresent so older CLI payloads that never emit it decode fine as nil).

Absence of the field always means "assume fresh," including for payloads from a CLI version that predates it — the fail-safe direction for a marker like this.

Follow-up (not in this PR)

The macOS menubar UI has no visible staleness indicator yet — this PR is data-model only for the Swift side. A subtle indicator (small dot/tooltip near the header, matching the pattern of #1096's cold-state signaling) is a reasonable small follow-up once there's a view spot picked for it; scoped out here to keep this change minimal and reviewable.

Test plan

• tests/menubar-json.test.ts: new cases assert stale: true when passed, and stale is undefined (not false) on a normal build. Verified revert-proof — reverting the src/menubar-json.ts + src/usage-aggregator.ts changes fails the new stale:true test.
• tests/usage-aggregator.test.ts: new assertion that a normal (env-isolated, always-complete) parse leaves payload.stale undefined.
• npm run test (full suite, excluding the separately-gated lock tests): 225 files / 3141 tests passed, 2 files / 5 tests skipped (pre-existing skips, unrelated).
• tsc --noEmit clean on both the CLI (tsconfig.json) and the desktop renderer (app/tsconfig.json).
• swift build and swift test --filter MenubarPayloadCombinedTests clean in mac/.

[iamtoruk]

Pushed a fix for the review's stale-global-read bug (commit e19c099).

Bug: isSessionHydrationComplete() reads a module-level global in parser.ts that's only safe to read immediately after the parse it describes. The prior commit read it at the very end of buildMenubarPayloadForRange, after several awaits — including, on the claude-config-scoped branch, a second parseAllSessions call (365-day history backfill) that runs after the parse producing the headline data. That let the history parse's hydration outcome silently overwrite the headline parse's.

Fix: capture isSessionHydrationComplete() into a local immediately after each branch's primary parse resolves (the scoped branch's Claude parse, or buildDurablePeriod on the non-scoped path), and thread that captured value to buildMenubarPayload instead of re-reading the global at the bottom. Also closes the cross-request race the review flagged (web-dashboard SWR / parallel MCP calls), since a synchronously-captured local can't be overwritten by another request's later parse.

New test (tests/usage-aggregator-freshness.test.ts): mocks parseAllSessions/isSessionHydrationComplete so the primary parse reports complete and a second (bystander) parse reports incomplete, then asserts the payload reflects the primary outcome. Verified it fails against the pre-fix code (expected true to be undefined, i.e. the bug reproduces) and passes against the fix.

Verified: full suite (226 files / 3142 tests passed, 2 files / 5 skipped — pre-existing), tsc --noEmit clean, swift build + swift test --filter MenubarPayloadCombinedTests clean.