fix(0123): wrong arg counts to fixed-arity fns error at the call site

checkCallArity compares the supplied count against the declared params
(min = params without trailing defaults, max = params.len, unbounded
past a variadic) at the five plain dispatch sites in lowerCall — bare
selected-author + lazy, namespace alias-gate + qualified, struct
method, ufcs. Pack / comptime / generic / #compiler / #builtin callees
keep their own dispatch. The method/ufcs sites also gain the
appendDefaultArgs fill the generic-instance leg already had, so
trailing defaults work on dot-calls instead of emitting under-arity
calls. lowerStmt's local fn_decl arm now registers a pointer into the
AST node in fn_ast_map, not a stack temporary that aliased every later
local fn.

src/ir/lower/stmt.zig

@@ -179,7 +179,10 @@ pub fn lowerStmt(self: *Lowering, node: *const Node) void {
     switch (node.data) {
         .var_decl => |vd| self.lowerVarDecl(&vd),
         .const_decl => |cd| self.lowerConstDecl(&cd),
-        .fn_decl => |fd| self.lowerLocalFnDecl(&fd),
+        // Pointer capture, not by-value: `lowerLocalFnDecl` registers the
+        // decl pointer in `fn_ast_map`, so it must point into the AST node,
+        // not at a stack temporary that the next statement reuses.
+        .fn_decl => |*fd| self.lowerLocalFnDecl(fd),
         .return_stmt => |rs| self.lowerReturn(&rs),
         .raise_stmt => |rs| self.lowerRaise(&rs, node.span),
         .assignment => |asgn| self.lowerAssignment(&asgn),