v1.3.2: Leaf cache for instant pin/unpin + TopPlans block UX

Decision-tree leaf outcomes are now cached on the main thread keyed by
their full 12-course assignment. Pin operations filter the cache and
re-derive top-K + per-set ceilings instantly with no worker spawn. Unpin
operations show the cached subset immediately and stream improvements as
a background worker fills in the missing leaves. Cache survives pin,
unpin, and adopt-plan; only ranking or mode changes invalidate it.

Solver / worker:

- searchDecisionTree accepts skipKeys (Set<string>) and pinnedAssignments
  (Record<setId,courseId>). Leaves are emitted with their full 12-set
  assignment so cache keys are stable across pin/unpin operations.
- evaluateLeaf short-circuits when the leaf's assignmentKey is in
  skipKeys: increments iterations + emits progress, but skips the
  optimizer call and all callbacks. Keeps progress percentage honest
  (counts whole tree, not just delta).
- New deriveFromLeaves pure helper produces {topK, setAnalyses} from a
  leaf collection; used by the main-thread cache filter and gives a
  reusable derivation primitive for tests.
- Worker request gains skipKeys and pinnedAssignments fields. Worker
  response gains a leafEvaluated event so the main thread can populate
  its cache as the search streams.

App state:

- leafCacheRef holds Map<assignmentKey, PlanOutcome> scoped to the
  current (ranking, mode) pair. The search effect now: invalidates on
  ranking/mode change; computes the orderedCourses + expectedTotal;
  filters the cache against the current pinned/excluded state; calls
  deriveFromLeaves to render immediately; spawns the worker only when
  filtered.length < expectedTotal, passing skipKeys.
- Cache cap of 500,000 leaves with full clear on overflow. Bounds
  worst-case memory at ~150 MB.

UI (TopPlans):

- Course blocks in the per-plan row are now interactive buttons. Click
  pins (or unpins, if the course is currently pinned) the course in
  that set. Pinned blocks render in a selected blue color.
- Each plan row now shows the FULL 12-set sequence including pinned
  courses (interleaved with the search's recommended choices for the
  remaining open sets) so the displayed plan is always complete.
- Spec qualification tags removed from per-block display (kept the
  set-label + course-name treatment for clarity).

Tests:

- New app/src/solver/__tests__/leafCache.test.ts with 4 tests:
  skipKeys parity (second-pass run with skipKeys evaluates zero
  leaves), deriveFromLeaves parity (matches a fresh search), cache
  filter on pinned assignments, cache filter on excluded courses.
- All 78 prior tests continue to pass; 82 total.

Browser-verified: pin click on a Top Plans block from the cached
8-open-set scenario completes instantly with no spinner; unpin restores
the original cached subset (also instant when the prior space was
already cached); mode toggle correctly invalidates and re-runs the
search.

openspec/changes/decision-tree-leaf-cache/specs/optimization-engine/spec.md

@@ -0,0 +1,63 @@
+## ADDED Requirements
+
+### Requirement: Persistent leaf cache across pin and unpin operations
+The application SHALL maintain a main-thread cache of evaluated decision-tree leaves keyed by the leaf's `assignmentKey` (the deterministic sorted `setId:courseId` join already used as the comparator tiebreaker). The cache SHALL persist across pin, unpin, and adopt-plan operations as long as `state.ranking` and `state.mode` are unchanged. Each cache entry SHALL store the full `PlanOutcome` (`courseAssignments`, `achievedSpecs`, `priorityScore`).
+
+#### Scenario: Pin operation hits cache fully
+- **WHEN** the user has completed a search with no pins on a small scenario, then pins a course
+- **THEN** the new top-K and per-set ceilings are derived entirely from the cache without spawning a worker
+- **AND** no "searching" indicators appear in the UI
+
+#### Scenario: Cache survives consecutive pin clicks
+- **WHEN** the user pins multiple courses one after another (or via "Adopt plan")
+- **THEN** every pin produces an instant UI update sourced from the existing cache
+
+#### Scenario: Unpin gets immediate cached subset and streams improvements
+- **WHEN** the user unpins a course after a search has populated the cache
+- **THEN** the UI immediately renders top-K and per-set ceilings derived from the cache subset matching the new state
+- **AND** a worker spawns to compute the missing leaves
+- **AND** as the worker streams new leaves, the UI's top-K and ceilings improve monotonically
+
+### Requirement: `skipKeys` worker contract
+The worker request SHALL accept an optional `skipKeys: string[]` field. The worker SHALL convert this list to a `Set<string>` and pass it to `searchDecisionTree`. Inside `evaluateLeaf`, leaves whose `assignmentKey` is in `skipKeys` SHALL be skipped: the optimizer SHALL NOT be invoked, no `topKUpdate` or `choiceUpdate` event SHALL be emitted for them, and the leaf SHALL NOT mutate per-set `evaluated` flags. Skipped leaves SHALL still increment the iteration counter so that throttled `progress` events report the total tree size, not just the delta.
+
+#### Scenario: Worker bypasses optimizer for cached leaves
+- **WHEN** the worker receives a request with `skipKeys` containing the keys of N cached leaves
+- **THEN** the worker performs at most `(iterationsTotal − N)` optimizer evaluations
+
+#### Scenario: Progress reports total tree size
+- **WHEN** the worker is processing a request with `skipKeys` containing 50,000 keys out of an `iterationsTotal` of 200,000
+- **THEN** progress events include `iterations` counting up to 200,000 (not 150,000) so the displayed percentage reflects whole-tree progress
+
+### Requirement: Cache invalidation on ranking, mode, or data change
+The leaf cache SHALL be cleared when `state.ranking` changes, when `state.mode` changes, or when the underlying course/specialization data is changed (e.g., a course is marked cancelled). Pin/unpin operations SHALL NOT trigger cache invalidation.
+
+#### Scenario: Mode toggle clears cache
+- **WHEN** the user toggles between maximize-count and priority-order
+- **THEN** the cache is emptied and the next search runs as a full recomputation
+
+#### Scenario: Ranking re-order clears cache
+- **WHEN** the user reorders the specialization ranking
+- **THEN** the cache is emptied and the next search runs as a full recomputation
+
+#### Scenario: Pin does not clear cache
+- **WHEN** the user pins or unpins a course
+- **THEN** the cache retains all previously evaluated leaves
+
+### Requirement: Cache size cap
+The leaf cache SHALL be cleared when its size exceeds 500,000 entries. Subsequent searches SHALL repopulate the cache from scratch.
+
+#### Scenario: Cap clears cache when exceeded
+- **WHEN** the cache is at 500,000 entries and a new search would add at least one more entry
+- **THEN** the cache is emptied before the next entry is inserted, and the new search proceeds without `skipKeys`
+
+### Requirement: `deriveFromLeaves` shared helper
+The decision-tree module SHALL export a pure function `deriveFromLeaves(leaves, K, mode, ranking, openSetIds, excludedCourseIds): { topK, setAnalyses }` that produces the top-K plan list and per-set ceiling table from a collection of leaf outcomes. This helper SHALL be used both by the worker at `allComplete` and by the main thread when rendering filtered cache results.
+
+#### Scenario: Helper output matches a fresh search
+- **WHEN** `deriveFromLeaves` is called with the complete leaf set from a finished `searchDecisionTree` run
+- **THEN** the returned `topK` and `setAnalyses` match the values that the search itself returned (modulo deterministic tiebreaker stability)
+
+#### Scenario: Helper output is correct for filtered subsets
+- **WHEN** `deriveFromLeaves` is called with a strict subset of cached leaves matching the user's current pinned/excluded state
+- **THEN** the returned top-K and ceilings reflect only those leaves and never reference courses outside the filter