ocaml: phase 5 HM pattern-match inference (+5 tests, 344 total)

ocaml-infer-pat covers :pwild, :pvar, :plit, :pcons, :plist, :ptuple,
:pas. Returns {:type T :env ENV2 :subst S} where ENV2 has the pattern's
bound names threaded through.

ocaml-infer-match unifies each clause's pattern type with the scrutinee,
runs the body in the env extended with pattern bindings, and unifies
all body types via a fresh result tv.

Examples:
  fun lst -> match lst with | [] -> 0 | h :: _ -> h : Int list -> Int
  match (1, 2) with | (a, b) -> a + b              : Int

Constructor patterns (:pcon) fall through to a fresh tv for now —
proper handling needs a ctor type registry from 'type' declarations.

plans/ocaml-on-sx.md

@@ -365,6 +365,14 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 5 — HM pattern-matching inference (+5 tests, 344
+  total). `ocaml-infer-pat` covers wild, var, lit, cons, list, tuple,
+  as. `ocaml-infer-match` unifies each clause's pattern type with the
+  scrutinee, runs the body in the env extended with pattern-bound vars,
+  and unifies all body types via a fresh result tv. Examples:
+  `fun lst -> match lst with | [] -> 0 | h :: _ -> h : Int list -> Int`.
+  Constructor patterns fall through to a fresh tv for now (need a ctor
+  type registry from `type` decls — pending).
 - 2026-05-08 Phase 6 — `List.sort` + polymorphic `compare` (+7 tests,
   339 total). `compare` is a host primitive that returns -1/0/1 like
   Stdlib.compare, defers to host SX `<`/`>`. `List.sort` is implemented