daily-cache: document coverage-widening and rebuild-projects limits; cover empty-call turns

Review addendum: the one-day tz widening assumes an offset delta of at
most 24h (date-line crossings are a known limit); day-level projects of
skinny kept slices are dropped by the coverage rebuild by design. Adds a
regression test pinning the existing skip of empty-call turns.

docs/lanes/carry-accounting-01.md

@@ -286,3 +286,50 @@ src/types.ts:190:  skillBreakdown: Record<string, { turns: number; costUSD: numb
 ```
 
 STATUS: BLOCKED (`npx vitest run` exits 1: 48 pre-existing discovery/import suite failures plus unrelated 5-second CLI timeouts; tried isolated timeout reproduction, a clean HEAD-plus-lane snapshot with locked app dependencies and one worker, and all clean root tests with a 15-second diagnostic timeout)
+
+## ADDENDUM
+
+### Touched files
+
+| file | added | deleted |
+|---|---:|---:|
+| `src/daily-cache.ts` | 8 | 1 |
+| `tests/day-aggregator.test.ts` | 42 | 0 |
+
+### New test
+
+| file | test |
+|---|---|
+| `tests/day-aggregator.test.ts` | `skips a timestamped empty-call turn without throwing or counting it at day level` |
+
+### `git diff --exit-code e07f324 -- src/day-aggregator.ts`
+
+Exit code: `0`
+
+```text
+```
+
+### `npx vitest run tests/daily-cache-carry-forward.test.ts tests/day-aggregator.test.ts`
+
+Exit code: `0`
+
+```text
+ RUN  v3.2.6 /Volumes/T8/Claude Projects/codeburn
+
+ ✓ tests/day-aggregator.test.ts (13 tests) 16ms
+ ✓ tests/daily-cache-carry-forward.test.ts (45 tests) 318ms
+
+ Test Files  2 passed (2)
+      Tests  58 passed (58)
+   Start at  17:14:56
+   Duration  740ms (transform 99ms, setup 49ms, collect 115ms, tests 334ms, environment 0ms, prepare 158ms)
+```
+
+### `npx tsc --noEmit`
+
+Exit code: `0`
+
+```text
+```
+
+STATUS: COMPLETE

src/daily-cache.ts

@@ -613,7 +613,10 @@ function setOwn<T>(target: Record<string, T>, key: string, value: T): void {
 /// provider's oldest fresh day (or the preceding day for timezone changes).
 export type SecondaryCoveragePolicy = {
   /// A timezone change can move a turn to an adjacent calendar day, so the day
-  /// immediately before the fresh provider floor is covered as well.
+  /// immediately before the fresh provider floor is covered as well. This
+  /// assumes the old/new timezone offset delta is at most 24 hours. Crossing
+  /// the date line can produce a 26-hour delta (UTC+14 to UTC-12) and move a
+  /// bucket by two calendar days; that rare case is a known, accepted limit.
   widenByOneDay: boolean
 }
 
@@ -669,6 +672,10 @@ export function mergeDayEntries(
       // Coverage can suppress one provider while another provider on the same
       // secondary-only day still needs carrying. Rebuild that day exclusively
       // from eligible slices so blocked totals cannot leak through day fields.
+      // Day-level projects cannot be attributed per provider: when a kept skinny
+      // slice has no slice.projects, its old day.projects entries are dropped.
+      // Copying them wholesale could restore the blocked provider's project
+      // totals, so this conservative loss is intentional.
       const copy = emptyDay(day.date)
       for (const [provider, slice] of providerEntries) {
         if ((!hasSliceData(slice) && !(slice.sessions ?? 0)) || !mayCarry(day.date, provider, slice)) continue

tests/day-aggregator.test.ts

@@ -168,6 +168,48 @@ describe('aggregateProjectsIntoDays', () => {
     })
   })
 
+  it('skips a timestamped empty-call turn without throwing or counting it at day level', () => {
+    const projects: ProjectSummary[] = [
+      makeProject({
+        sessions: [{
+          sessionId: 'empty-call-turn',
+          project: 'p',
+          firstTimestamp: '2026-04-09T10:00:00',
+          lastTimestamp: '2026-04-09T10:00:00',
+          totalCostUSD: 0,
+          totalInputTokens: 0,
+          totalOutputTokens: 0,
+          totalCacheReadTokens: 0,
+          totalCacheWriteTokens: 0,
+          apiCalls: 0,
+          turns: [{
+            userMessage: 'no assistant response',
+            timestamp: '2026-04-09T10:00:00',
+            sessionId: 'empty-call-turn',
+            category: 'coding',
+            retries: 0,
+            hasEdits: true,
+            assistantCalls: [],
+          }],
+          modelBreakdown: {},
+          toolBreakdown: {},
+          mcpBreakdown: {},
+          bashBreakdown: {},
+          categoryBreakdown: {} as never,
+          skillBreakdown: {} as never,
+        }],
+      }),
+    ]
+
+    let days: ReturnType<typeof aggregateProjectsIntoDays> = []
+    expect(() => { days = aggregateProjectsIntoDays(projects) }).not.toThrow()
+
+    expect(days).toHaveLength(1)
+    expect(days[0]).toMatchObject({ cost: 0, calls: 0, editTurns: 0, oneShotTurns: 0 })
+    expect(days[0]!.categories).toEqual({})
+    expect(days[0]!.providers).toEqual({})
+  })
+
   it('counts a session under its firstTimestamp date', () => {
     const projects: ProjectSummary[] = [
       makeProject({