apl: ⍎ execute — eval string as APL source (+8)

- apl-execute: reassemble char-vector ravel into single string,
  then apl-run; handles plain string, scalar, and char-vector
- nested ⍎ ⍎ works; ⋄ separator threads through
- pipeline 148/148

plans/apl-on-sx.md

@@ -255,7 +255,7 @@ still need work to run as-written.  Phase 10 closes both.
   and 0 cells are dropped.  Returns a vector of (boxed) partitions.
   Add `apl-partition`.  Tests: `1 1 0 1 1 ⊆ 'abcde' → ('ab' 'de')`,
   `1 0 0 1 1 ⊆ ⍳5 → ((⊂ 1) (⊂ 4 5))`.
-- [ ] **`⍎` execute** — monadic `⍎ S` evaluates `S` (a character
+- [x] **`⍎` execute** — monadic `⍎ S` evaluates `S` (a character
   vector) as APL source in the *current* environment, returning the
   result.  Implement as `(fn (s) (apl-run s))` — env is the global
   one; nested execute is fine.  Wire into `apl-monadic-fn`.  Tests:
@@ -288,6 +288,7 @@ data; format for string templating.
 
 _Newest first._
 
+- 2026-05-08: Phase 10 step 5 — `⍎` execute. apl-execute reassembles char-vector ravel into single string then calls apl-run; handles plain string, scalar, and char-vector. `⍎ '1 + 2' → 3`, `⍎ '+/⍳10' → 55`, round-trip `⍎ ⎕FMT 42 → 42`, nested `⍎ ⍎ '...'` works, with `⋄` separator (assignment + use). Wired into apl-monadic-fn. +8 tests; pipeline 148/148
 - 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