Step 7d complete: exhaustive match checking + evaluator cleanup

Match exhaustiveness analysis:
- check-match-exhaustiveness function in evaluator.sx
- lint-node in tree-tools.sx checks match forms during format-check
- Warns on: no wildcard/catch-all, boolean missing true/false case
- (match x (true "yes")) → "match may be non-exhaustive"

Evaluator cleanup:
- Added missing step-sf-callcc definition (was in old transpiled output)
- Added missing step-sf-case definition (was in old transpiled output)
- Removed protocol functions from bootstrap skip set (they transpile fine)
- Retranspiled VM (bootstrap_vm.py) for compatibility

2650 tests pass (+5 from new features).

All Step 7 features complete:
  7a: ->> |> as-> pipe operators
  7b: Dict patterns, &rest, let-match destructuring
  7c: define-protocol, implement, satisfies?
  7d: Exhaustive match checking

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/bootstrap.py

@@ -53,6 +53,7 @@ let trampoline v = !trampoline_fn v
 let _strict_ref = ref (Bool false)
 let _prim_param_types_ref = ref Nil
 let _last_error_kont_ref = ref Nil
+let _protocol_registry_ = Dict (Hashtbl.create 0)
 
 """
 
@@ -209,6 +210,15 @@ def compile_spec_to_ml(spec_dir: str | None = None) -> str:
     # the transpiler directly — it emits !_ref for reads, _ref := for writes.
     import re
 
+    # Remove `and _protocol_registry_ = (Dict ...)` from the let rec block —
+    # it's defined in the preamble as a top-level let, and Hashtbl.create
+    # is not allowed as a let rec right-hand side.
+    output = re.sub(
+        r'\n\(\* \*protocol-registry\*.*?\nand _protocol_registry_ =\n  \(Dict \(Hashtbl\.create 0\)\)\n',
+        '\n',
+        output
+    )
+
     return output