feat: capture-less anonymous functions (closures)

Implement MVP closures — anonymous functions without captures.

Syntax:
  |a: int, b: int| -> int { return a + b; }

Changes:
- ast.bux + ast.nim: add ekClosure AST node
- parser.bux + parser.nim: parse |params| -> Ret { body }
- sema.bux + sema.nim: type-check closure params/body, return tyFunc
- hir_lower.bux + hir_lower.nim: generate __closure_N function + hAddrOf
- lir_c_backend.nim: fix function-pointer variable declaration (cParamDecl)
- C backend: closures compile to global functions with unique names

Test: _test_closure/src/Main.bux
- Closure as variable
- Closure passed to higher-order function
- Address of named function as function pointer

Both bootstrap and selfhost compilers build and pass the test.

bootstrap/ast.nim

@@ -132,6 +132,7 @@ type
     ekBlock
     ekMatch
     ekStringInterp
+    ekClosure
 
   MatchArm* = object
     loc*: SourceLocation
@@ -228,6 +229,10 @@ type
     of ekStringInterp:
       exprInterpTexts*: seq[string]
       exprInterpExprs*: seq[Expr]
+    of ekClosure:
+      exprClosureParams*: seq[Param]
+      exprClosureBody*: Block
+      exprClosureReturnType*: TypeExpr
 
   # ---------------------------------------------------------------------------
   # Statements