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/lir_c_backend.nim

@@ -58,6 +58,13 @@ proc typeFromValue(be: var LirCBackend, v: LirValue): string =
 proc setTempType(be: var LirCBackend, temp: string, cType: string) =
   be.tempTypes[temp] = cType
 
+proc cParamDecl(cType, name: string): string =
+  ## Emit a C parameter declaration, handling function-pointer syntax.
+  if cType.contains("(*)"):
+    return cType.replace("(*)", "(*" & name & ")")
+  else:
+    return cType & " " & name
+
 # ── Per-instruction emission ──
 
 proc emitInstr(be: var LirCBackend, instr: LirInstr) =
@@ -183,7 +190,7 @@ proc emitInstr(be: var LirCBackend, instr: LirInstr) =
       let inferred = be.tempTypes[instr.dst.strVal]
       if inferred != "" and inferred != ct:
         ct = inferred
-    be.emitLine(&"{ct} {v(instr.dst)};")
+    be.emitLine(cParamDecl(ct, v(instr.dst)) & ";")
 
   # ── Pointers ──
   of lirAddrOf:
@@ -241,13 +248,6 @@ proc emitInstr(be: var LirCBackend, instr: LirInstr) =
 
 # ── Function emission ──
 
-proc cParamDecl(cType, name: string): string =
-  ## Emit a C parameter declaration, handling function-pointer syntax.
-  if cType.contains("(*)"):
-    return cType.replace("(*)", "(*" & name & ")")
-  else:
-    return cType & " " & name
-
 proc emitFunc(be: var LirCBackend, f: LirFunc, funcRetTypes: Table[string, string], funcPtrTypes: Table[string, string]) =
   var paramsStr = ""
   for i, p in f.params: