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.zig

@@ -1800,6 +1800,7 @@ pub const Lowering = struct {
     pub const reflectionTypeArgGuard = lower_call.reflectionTypeArgGuard;
     pub const reflectionErrorSentinel = lower_call.reflectionErrorSentinel;
     pub const appendDefaultArgs = lower_call.appendDefaultArgs;
+    pub const checkCallArity = lower_call.checkCallArity;
     pub const expandCallDefaults = lower_call.expandCallDefaults;
     pub const userParamTypes = lower_call.userParamTypes;
     pub const resolveCallParamTypes = lower_call.resolveCallParamTypes;