Auto: focas-f5a — cycle time per part / last cycle delta

Closes #272

docs/v2/implementation/focas-simulator-plan.md

@@ -304,6 +304,82 @@ Bit-level writes never appear here as a separate kind — they reach the
 simulator as 1-byte writes after the driver's RMW wrapper, so the audit
 shape is identical to a byte write at the same address.
 
+## Cycle-time per part / last cycle delta — F5-a (issue #272)
+
+Plan PR F5-a derives `Production/LastCycleSeconds` +
+`Production/LastCycleStartUtc` from the existing `cnc_rdparam(6711)` +
+`cnc_rdtimer` snapshot stream — **pure derivation, no new wire calls**.
+The simulator does NOT need new wire commands; the existing
+`cnc_rdparam` + `cnc_rdtimer` handlers already cover the read surface.
+
+What focas-mock DOES need is an admin endpoint + test-fixture helper
+that lets integration tests atomically increment the parts-count
+counter alongside the cycle-time timer so the driver sees a clean
+"cycle completed" transition on the next probe tick.
+
+### Per-profile state
+
+Already covered by the existing F1-b state map:
+
+- `parameters: Dict[int, int]` (entry `6711` is the parts-count counter).
+- `timers: Dict[int, int]` (entry `0` is the live cycle-time counter,
+  in seconds).
+
+### Admin endpoint — `POST /admin/mock_simulate_cycle_completion`
+
+Atomically advances both values to model "the CNC just finished a
+cycle". Atomicity matters: the F5-a derivation samples both fields on
+every probe tick, so if the simulator updated parts-count and the
+timer in two separate writes the test could observe an intermediate
+state where parts-count incremented but the timer hasn't updated yet
+(producing a misleading `LastCycleSeconds`).
+
+```
+POST /admin/mock_simulate_cycle_completion
+{
+  "profile": "Series30i",
+  "partsDelta": 1,                  // default 1; tests asserting backfill use 3+
+  "newCycleTimerSeconds": 18        // absolute value, NOT a delta
+}
+```
+
+Handler steps:
+
+1. `parameters[6711] += partsDelta` (under the per-profile lock).
+2. `timers[0] = newCycleTimerSeconds`.
+3. Return `200 OK` with the new values for verification.
+
+The endpoint MUST hold the profile's update lock for the full
+read-modify-write so a concurrent `cnc_rdparam` + `cnc_rdtimer` poll
+sees both fields in their pre-update OR post-update state — never
+half-applied.
+
+### `FocasSimFixture.SimulateCycleCompletionAsync`
+
+The future test-support helper wraps the admin endpoint:
+
+```csharp
+await fixture.SimulateCycleCompletionAsync(
+    profile: "Series30i",
+    partsDelta: 1,
+    newCycleTimerSeconds: 18);
+```
+
+Integration test `Series/CycleDeltaTests.cs` will assert:
+
+- After a 5 -> 6 transition with `newCycleTimerSeconds=18`, the
+  driver's `Production/LastCycleSeconds` settles to `currentTimer -
+  prevTimer`.
+- `Production/LastCycleStartUtc` is within driver-tolerance of
+  `nowUtc - LastCycleSeconds` (allow a small window for probe-tick
+  jitter).
+- Counter reset (parts -> 0) preserves the last published values.
+- Cycle-timer rollover does not publish a negative delta.
+
+These tests are blocked on the focas-mock + integration-test project
+landing; the unit-test coverage in `FocasCycleDeltaTests` already
+exercises every same-process invariant of the derivation.
+
 ### Status
 
 focas-mock simulator has not landed yet (tracked separately from F4-b /