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.exit

@@ -0,0 +1 @@
+0

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

@@ -0,0 +1 @@
+

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

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

examples/expected/1167-diagnostics-call-arity-mismatch.exit

@@ -0,0 +1 @@
+1

examples/expected/1167-diagnostics-call-arity-mismatch.stderr

@@ -0,0 +1,29 @@
+error: 'add2' expects 2 arguments, but 3 were given
+  --> examples/1167-diagnostics-call-arity-mismatch.sx:19:9
+   |
+19 |     _ = add2(1, 2, 3);           // plain bare call, too many
+   |         ^^^^
+
+error: 'add2' expects 2 arguments, but 1 was given
+  --> examples/1167-diagnostics-call-arity-mismatch.sx:20:9
+   |
+20 |     _ = add2(1);                 // plain bare call, too few
+   |         ^^^^
+
+error: 'concat' expects 2 arguments, but 3 were given
+  --> examples/1167-diagnostics-call-arity-mismatch.sx:21:9
+   |
+21 |     _ = concat("a", "b", "c");   // flat-imported stdlib fn, too many
+   |         ^^^^^^
+
+error: 'Point.scaled' expects 1 argument, but 2 were given
+  --> examples/1167-diagnostics-call-arity-mismatch.sx:23:9
+   |
+23 |     _ = p.scaled(2, 9);          // method dot-call, too many
+   |         ^^^^^^^^
+
+error: 'bump' expects 1 argument, but 2 were given
+  --> examples/1167-diagnostics-call-arity-mismatch.sx:24:9
+   |
+24 |     _ = p.bump(1, 2);            // ufcs dot-call, too many
+   |         ^^^^^^

examples/expected/1167-diagnostics-call-arity-mismatch.stdout

@@ -0,0 +1 @@
+