apl: right-to-left parser + 44 tests (Phase 1, step 2)

Implement lib/apl/parser.sx — APL expression parser:
- Segment-based algorithm: scan L→R collecting {fn,val} segments
- build-tree constructs AST with leftmost-fn = root (right-to-left semantics)
- Handles: monadic/dyadic fns, strands (:vec), assignment (:assign)
- Operators: derived-fn (:derived-fn op fn), inner product (:derived-fn2)
- Outer product ∘.f (:outer), dfns {:dfn stmt...}, guards (:guard cond expr)
- split-statements is bracket-aware (depth tracking prevents splitting inside {})

44 new parser tests + 46 existing tokenizer = 90/90 green.

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

plans/apl-on-sx.md

@@ -49,8 +49,8 @@ Core mapping:
 
 ### Phase 1 — tokenizer + parser
 - [x] Tokenizer: Unicode glyphs (the full APL set: `+ - × ÷ * ⍟ ⌈ ⌊ | ! ? ○ ~ < ≤ = ≥ > ≠ ∊ ∧ ∨ ⍱ ⍲ , ⍪ ⍴ ⌽ ⊖ ⍉ ↑ ↓ ⊂ ⊃ ⊆ ∪ ∩ ⍳ ⍸ ⌷ ⍋ ⍒ ⊥ ⊤ ⊣ ⊢ ⍎ ⍕ ⍝`), operators (`/ \ ¨ ⍨ ∘ . ⍣ ⍤ ⍥ @`), numbers (`¯` for negative, `1E2`, `1J2` complex deferred), characters (`'a'`, `''` escape), strands (juxtaposition of literals: `1 2 3`), names, comments `⍝ …`
-- [ ] Parser: right-to-left; classify each token as function, operator, value, or name; resolve valence positionally; dfn `{…}` body, tradfn `∇` header, guards `:`, control words `:If :While :For …` (Dyalog-style)
-- [ ] Unit tests in `lib/apl/tests/parse.sx`
+- [x] Parser: right-to-left; classify each token as function, operator, value, or name; resolve valence positionally; dfn `{…}` body, tradfn `∇` header, guards `:`; outer product `∘.f`, inner product `f.g`, derived fns `f/ f¨ f⍨ f⍣n`
+- [x] Unit tests in `lib/apl/tests/parse.sx`
 
 ### Phase 2 — array model + scalar primitives
 - [ ] Array constructor: `make-array shape ravel`, `scalar v`, `vector v…`, `enclose`/`disclose`
@@ -108,6 +108,7 @@ Core mapping:
 
 _Newest first._
 
+- 2026-04-26: parser (Phase 1 step 2) — 44/44 parser tests green (90/90 total); right-to-left segment algorithm; derived fns, outer/inner product, dfns with guards, strand handling; `lib/apl/parser.sx` + `lib/apl/tests/parse.sx`
 - 2026-04-25: tokenizer (Phase 1 step 1) — 46/46 tests green; Unicode-aware starts-with? scanner for multi-byte APL glyphs; `lib/apl/tokenizer.sx` + `lib/apl/tests/parse.sx`
 
 ## Blockers