## Why

After v1.3.1 made the decision-tree search exhaustive, every pin/unpin click triggers a fresh full search (~7s in the worker for the user's typical 8-open-set scenario). That re-run is wasteful: a leaf's outcome (`achievedSpecs`, `priorityScore`) depends only on its 12-course assignment plus ranking and mode. Pinning or unpinning a course never changes the value of any leaf that has already been evaluated; it only changes which leaves are reachable in the current search.

We should cache evaluated leaves and re-derive the top-K and per-set ceilings from the cache when pin/unpin operations leave ranking and mode untouched. Pin operations become 100% cache hits (instant). Unpin operations get a partial cache hit — show the cached subset immediately as a lower-bound result, then run the worker to fill in only the leaves that haven't been evaluated yet.

## What Changes

- Add a main-thread leaf cache: `Map<assignmentKey, PlanOutcome>` keyed by the existing `assignmentKey` (sorted setId:courseId join). Cache instance is held in a ref on `useAppState`.
- Cache invalidation triggers: any change to `state.ranking`, `state.mode`, or the cancellation list (data version). Pin, unpin, and adopt-plan operations leave the cache intact.
- New effect logic in `useAppState`: when the search-effect dependencies change, first FILTER the existing cache against the new pinned/excluded state and derive top-K + per-set ceilings from the filtered subset, rendering immediately. Then check whether the filtered count equals the expected cartesian-product size; if not, spawn a worker to compute the missing leaves only.
- Add an optional `skipKeys: Set<string>` parameter to `searchDecisionTree` and to the worker's `WorkerRequest`. In `evaluateLeaf`, leaves whose `assignmentKey` is in `skipKeys` are skipped entirely — no optimizer call, no callback emit, no iteration counted toward progress (or counted but not evaluated; design choice in tasks).
- The worker streams new leaves the same way it streams `topKUpdate` / `choiceUpdate` events today; main thread inserts each new leaf into the cache as it arrives.
- Soft cap: if the cache exceeds **500,000** entries, clear it entirely. Subsequent searches recompute from scratch. The cap exists only to bound worst-case memory; typical exploration paths never approach it.
- "Searching" UI indicators (per-set spinner, global progress bar) only appear when the worker actually runs (i.e., not on full-cache-hit pin clicks).

## Capabilities

### New Capabilities

_None._

### Modified Capabilities

- `optimization-engine`: introduces leaf caching across search invocations, the `skipKeys` worker contract, the immediate-render-then-stream pipeline for partial cache hits, and the cache-cap eviction policy. The optimizer and LP feasibility checker are untouched.

## Impact

- `app/src/state/appState.ts` — add `leafCacheRef` (Map<string, PlanOutcome>); restructure the worker effect to filter cache, derive immediate state, and spawn the worker only for the delta. Reset cache on ranking/mode change. Apply the 500k cap.
- `app/src/solver/decisionTree.ts` — add `skipKeys?: Set<string>` to `SearchCallbacks`/`searchDecisionTree` signature; in `evaluateLeaf` short-circuit when the leaf's assignmentKey is already in `skipKeys`. Export a `deriveFromLeaves(leaves, K, mode, ranking, openSets, excludedCourseIds)` helper that produces `{ topK, setAnalyses }` from a leaf collection, used both by the worker (final emit) and the main-thread filter path.
- `app/src/workers/decisionTree.worker.ts` — accept `skipKeys?: string[]` in `WorkerRequest`; convert to `Set<string>` and pass to `searchDecisionTree`.
- `app/src/solver/__tests__/searchDecisionTree.test.ts` — add tests: skipKeys correctly bypasses optimizer; `deriveFromLeaves` matches a fresh search's output when given the same leaves; cache filter pin/unpin idempotence (same final state regardless of pin order).
- New unit test file (or extension of existing): cache-cap eviction, ranking/mode invalidation.
- `app/vite.config.ts` — bump to `1.3.2`
- `CHANGELOG.md` — release entry
- No data-file changes