apl: ⊆ partition — mask-driven split (+8)

- apl-partition: new partition where M[i]>M[i-1] (init prev=0);
  continue where M[i]≤prev∧M[i]≠0; drop cells where M[i]=0
- Returns apl-vector of apl-vector parts
- pipeline 140/140

plans/apl-on-sx.md

@@ -250,7 +250,7 @@ still need work to run as-written.  Phase 10 closes both.
   conformable vector.  Add `apl-decode` and `apl-encode`.  Tests:
   `2 ⊥ 1 0 1 → 5`, `10 ⊥ 1 2 3 → 123`, `2 2 2 ⊤ 5 → 1 0 1`,
   `24 60 60 ⊤ 7384 → 2 3 4`.
-- [ ] **`⊆` partition** — dyadic `M ⊆ V` partitions `V` into vectors
+- [x] **`⊆` partition** — dyadic `M ⊆ V` partitions `V` into vectors
   driven by mask `M`: a new partition starts wherever `M[i] > M[i-1]`,
   and 0 cells are dropped.  Returns a vector of (boxed) partitions.
   Add `apl-partition`.  Tests: `1 1 0 1 1 ⊆ 'abcde' → ('ab' 'de')`,
@@ -288,6 +288,7 @@ data; format for string templating.
 
 _Newest first._
 
+- 2026-05-08: Phase 10 step 4 — `⊆` partition. apl-partition: walk M and V together via reduce, opening a new partition where M[i]>M[i-1] (initial prev=0), continuing where M[i]≤prev∧M[i]≠0, dropping cells where M[i]=0. Returns apl-vector of apl-vector parts. `1 1 0 1 1 ⊆ 'abcde' → ('ab' 'de')`, `1 0 0 1 1 ⊆ ⍳5 → ((1) (4 5))`, strict-increase `1 2` opens new, constant `2 2` continues. Wired into apl-dyadic-fn. +8 tests; pipeline 140/140
 - 2026-05-08: Phase 10 step 3 — `⊥` decode / `⊤` encode. apl-decode (Horner reduce over indices, base[i]>0; scalar base broadcasts to digit length); apl-encode (right-to-left modulo+floor-div via reduce). Mixed-radix HMS works: `24 60 60 ⊥ 2 3 4 → 7384`, `24 60 60 ⊤ 7384 → 2 3 4`. Round-trips exact. Wired ⊥ ⊤ into apl-dyadic-fn. +11 tests; pipeline 132/132
 - 2026-05-08: Phase 10 step 2 — `∪` unique / `∩` intersection. apl-unique (monadic, dedup keeping first-occurrence order via reduce+index-of), apl-union (dyadic, dedup'd A then B-elements-not-in-A), apl-intersect (dyadic, A elements that are also in B, preserves left order). Wired ∪ into both apl-monadic-fn and apl-dyadic-fn cond chains; ∩ into apl-dyadic-fn. +12 tests; pipeline 121/121
 - 2026-05-08: Phase 10 step 1 — `⍸` where. apl-where (monadic, indices of truthy cells, ⎕IO-respecting) + apl-interval-index (dyadic, count of breaks ≤ y; broadcasts over Y vector or scalar). Wired into apl-monadic-fn / apl-dyadic-fn (cond clauses inserted as proper siblings via sx_insert_child after sx_insert_near silently wrapped multi-form sources in `(begin …)`). +10 tests; pipeline 109/109