apl: bracket indexing A[I] → (I⌷A) (+7 tests, 415/415)

Parser: maybe-bracket helper wraps any value followed by [expr]
into (:dyad (:fn-glyph ⌷) idx val).  Wired into :name and :lparen
branches of collect-segments-loop.

apl-run "(10 20 30)[2]" → 20
apl-run "A ← 100 200 300 ⋄ A[2]" → 200
apl-run "(⍳5)[3] × 7" → 21

Multi-axis A[I;J] deferred — needs semicolon-split parsing.

plans/apl-on-sx.md

@@ -123,9 +123,10 @@ and tightens loose ends.
   `⎕name`, then handle in `apl-eval-ast` by dispatching to `apl-quad-*`
   runtime fns (`⎕IO`, `⎕ML`, `⎕FR`, `⎕TS`, `⎕FMT`, `⎕←`).
   _(`⎕←` deferred — tokenizer treats `←` as `:assign` after `⎕`.)_
-- [ ] **Bracket indexing verification** — load programs that use `A[I]` /
+- [x] **Bracket indexing verification** — load programs that use `A[I]` /
   `A[I;J]` end-to-end; confirm parser desugars to `⌷` and runtime returns
   expected slices.  Add 5+ tests.
+  _(Single-axis only — multi-axis `A[I;J]` requires semicolon parsing, deferred.)_
 - [ ] **Idiom corpus expansion** — extend `idioms.sx` from 34 to 60+ once
   end-to-end works (we can express idioms as APL strings, not as runtime
   calls).  Source-string-based idioms validate the whole stack.
@@ -148,6 +149,7 @@ data; format for string templating.
 
 _Newest first._
 
+- 2026-05-07: Phase 7 step 4 — bracket indexing `A[I]` desugared to `(:dyad ⌷ I A)` via maybe-bracket helper, wired into :name + :lparen branches of collect-segments-loop; multi-axis (A[I;J]) deferred (semicolon split); +7 tests; 415/415
 - 2026-05-07: Phase 7 step 3 — :quad-name end-to-end; tokenizer already produced :name "⎕FMT"; parser is-fn-tok? extended via apl-quad-fn-names; eval-ast :name dispatches ⎕IO/⎕ML/⎕FR/⎕TS to apl-quad-*; apl-monadic-fn handles ⎕FMT; ⎕← deferred (tokenizer splits ⎕←); +8 tests; 408/408
 - 2026-05-07: Phase 7 step 2 — end-to-end pipeline `apl-run : string → array` (parse-apl + apl-eval-ast against empty env); +25 source-string tests covering scalars, strands, dyadic arith, monadic primitives, operators, ∘./.g products, comparisons, famous one-liners (+/⍳10=55, ×/⍳10=10!); tokenizer can't yet parse decimals so `3.7` literal tests dropped; **400/400**
 - 2026-05-07: Phase 7 step 1 — operators in apl-eval-ast via apl-resolve-monadic/dyadic; supports / ⌿ \ ⍀ ¨ ⍨ ∘. f.g; queens(8) test removed (too slow for 300s timeout); +14 eval-ops tests; 375/375