haskell: Phase 17 — expression type annotations (x :: Int) (parse + desugar pass-through)

Parser hk-parse-parens gains a `::` arm after the first inner expression:
consume `::`, parse a type via the existing hk-parse-type, expect `)`,
emit (:type-ann EXPR TYPE). Sections, tuples, parenthesised expressions
and unit `()` are unchanged.

Desugar drops the annotation — :type-ann E _ → (hk-desugar E) — since
the existing eval path has no type-directed dispatch. Phase 20 will
extend infer.sx to consume the annotation and unify against the
inferred type.

tests/parse-extras.sx (12/12) covers literal, arithmetic, function arg,
string, bool, tuple, nested annotation, function-typed annotation, and
no-regression checks for plain parens / 3-tuples / left+right sections.
eval (66/0), exceptions (14/0), typecheck (15/0), records (14/0), ioref
(13/0) all still clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plans/haskell-completeness.md

@@ -316,7 +316,7 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 Real Haskell programs use these on every page; closing the gaps unblocks
 larger conformance programs and removes one-line workarounds in test sources.
 
-- [ ] Type annotations in expressions: `(x :: Int)`, `f (1 :: Int)`,
+- [x] Type annotations in expressions: `(x :: Int)`, `f (1 :: Int)`,
   `return (42 :: Int)`. Parser currently rejects `::` in `aexp` position;
   desugar should drop the annotation (we have no inference at this layer
   yet, so it's a parse-only pass-through).
@@ -420,6 +420,15 @@ constraints (qualified types `[ClassName var] => type`).
 
 _Newest first._
 
+**2026-05-08** — Phase 17 first box: expression type annotations `(x :: Int)`,
+`f (1 :: Int)`, `(\x -> x+1) :: Int -> Int`. Parser's `hk-parse-parens`
+gains a `::` arm after the first inner expression: consume `::`, parse a
+type via the existing `hk-parse-type`, expect `)`, emit `(:type-ann EXPR
+TYPE)`. Desugar drops the annotation — `:type-ann E _ → (hk-desugar E)` —
+since the existing eval path has no type-directed dispatch; Phase 20 will
+let inference consume the annotation. tests/parse-extras.sx 12/12; eval,
+exceptions, typecheck, records, ioref still clean.
+
 **2026-05-08** — Plan extends with Phases 20-22 (HM type system). Discovered
 during planning that `lib/haskell/infer.sx` already lands core Algorithm W
 (75 inference unit tests pass; let-polymorphism, sig checking, error