plans: tick Phase 3 squad ⌷, progress log

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plans/apl-on-sx.md

@@ -66,7 +66,7 @@ Core mapping:
 - [x] Reshape `⍴`, ravel `,`, transpose `⍉` (full + dyadic axis spec)
 - [x] Take `↑`, drop `↓`, rotate `⌽` (last axis), `⊖` (first axis)
 - [x] Catenate `,` (last axis) and `⍪` (first axis)
-- [ ] Index `⌷` (squad), bracket-indexing `A[I]` (sugar for `⌷`)
+- [x] Index `⌷` (squad), bracket-indexing `A[I]` (sugar for `⌷`)
 - [ ] Grade-up `⍋`, grade-down `⍒`
 - [ ] Enclose `⊂`, disclose `⊃`, partition (subset deferred)
 - [ ] Membership `∊`, find `⍳` (dyadic), without `~` (dyadic), unique `∪` (deferred to phase 6)
@@ -118,6 +118,7 @@ data; format for string templating.
 
 _Newest first._
 
+- 2026-05-06: Phase 3 step 4 — squad ⌷ (scalar/multi-dim/partial-slice); 66/66 tests
 - 2026-05-06: Phase 3 step 3 — catenate , (last axis, scalar promo) and first-axis; 59/59 tests
 - 2026-05-06: Phase 3 step 2 — take ↑ (multi-axis, pad), drop ↓, reverse/rotate ⌽⊖ (last+first axis); 50/50 tests
 - 2026-05-06: Phase 3 step 1 — reshape ⍴ (cycling), transpose ⍉ (monadic+dyadic); helpers apl-strides/flat->multi/multi->flat; 27/27 structural tests; lib/apl/tests/structural.sx