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.

examples/expected/0054-basic-dot-call-default-args.stdout

@@ -0,0 +1,7 @@
+10
+15
+15
+6
+0
+6
+true