S9 OR Tools basic scheduling

specs/S3_define-coding-standards-to-fe

@@ -0,0 +1 @@
+Create a file (or append to it) CODING_STANDARDS.md and have the coding standards in there, for the review skill. 

specs/S7_define-coding-standards-to-be

@@ -0,0 +1 @@
+Create a file (or append to it) CODING_STANDARDS.md and have the coding standards in there, for the review skill. 

specs/in_review/S9_spread-dancegroups-via-ortools-setup

@@ -1,6 +1,6 @@
 # S9 — Schedule dance groups across show dayparts with OR-Tools
 
-Status: REFINED
+Status: IN_REVIEW
 
 Agreed after the grill-me review; shared understanding confirmed.
 
@@ -121,33 +121,43 @@ S10–S13 are intentionally empty placeholders and are to be refined separately.
 
 ## Acceptance criteria
 
-- [ ] Configure up to six dated dayparts, reject duplicate date/type combinations
+- [x] Configure up to six dated dayparts, reject duplicate date/type combinations
       and derive the displayed show date range from them.
-- [ ] Persist editable per-type counts with the agreed defaults, remove the
+- [x] Persist editable per-type counts with the agreed defaults, remove the
       backend type-level property and validate counts in frontend and backend.
-- [ ] With one daypart, every count is one and frontend fields are disabled.
-- [ ] Daypart edits preserve configured counts and require correction of counts
+- [x] With one daypart, every count is one and frontend fields are disabled.
+- [x] Daypart edits preserve configured counts and require correction of counts
       that exceed the new number of dayparts.
-- [ ] OR-Tools assigns every school group to exactly its type's required number
+- [x] OR-Tools assigns every school group to exactly its type's required number
       of distinct dayparts and minimizes the busiest/quietest appearance gap.
-- [ ] Scheduling requires groups and dayparts and replaces assignments only after
+- [x] Scheduling requires groups and dayparts and replaces assignments only after
       a successful, current result.
-- [ ] The synchronous request supports a 30-minute solver limit, accepts valid
+- [x] The synchronous request supports a 30-minute solver limit, accepts valid
       unproven results and communicates whether optimality was proven.
-- [ ] Failed, infeasible, stale and no-result runs preserve existing assignments
+- [x] Failed, infeasible, stale and no-result runs preserve existing assignments
       and explain why no replacement was saved.
-- [ ] Duplicate concurrent runs for a show are prevented in frontend and backend.
-- [ ] Potentially invalidating edits warn beforehand, identify affected shows,
+- [x] Duplicate concurrent runs for a show are prevented in frontend and backend.
+- [x] Potentially invalidating edits warn beforehand, identify affected shows,
       permit cancellation and expose invalid schedules afterward without
       automatically rescheduling.
-- [ ] New-format JSON round-trips dates, assignments and all type settings;
+- [x] New-format JSON round-trips dates, assignments and all type settings;
       legacy import support is not required.
-- [ ] Any repeatability work respects both the three-minute investigation limit
+- [x] Any repeatability work respects both the three-minute investigation limit
       and the ten-additional-statement implementation limit.
 
 ## Implementation handoff
 
-This specification records agreed behavior; it does not implement scheduling.
+This specification records the agreed behavior implemented on the current branch.
 The current application uses in-memory storage with JSON import/export for
 transfer. S9 adds the scheduling model, frontend controls, backend operation and
 JSON changes within that existing persistence approach.
+
+## Implementation validation
+
+- Backend: 29 tests, including real OR-Tools balancing, input validation, JSON
+  round-trips, concurrent scheduling, stale input rejection and failure preservation.
+- Frontend: 20 tests covering count validation, disabled single-daypart fields, edit cancellation,
+  API timeout contract, production build and lint checks.
+- Playwright: real REST workflow for dated dayparts, scheduling and cancellation.
+- Repeatability uses one solver-parameter statement (single worker, fixed seed);
+  no extended investigation was needed.