fix(resolver): share plan SelectedFunc across consumers + route UFCS through selector [stdlib C attempt-2]

Address Phase C review (C-1, C-2): make CallResolver.plan's SelectedFunc the
single shared call author consumed by the lower-call sites instead of each
re-resolving; route free-fn value-receiver UFCS through the selector in plan so
plan typing and lowering pick the same author under a flat same-name collision.
Adds regression 0740-modules-flat-same-name-ufcs-typing.

Salvaged from a worker killed at the wall during its final gate step; manager
verified the gate at ground truth (zig build test exit 0; run_examples 476/0 with
0722-0735 + 0740 ok; m3te ios-sim exit 0).

examples/0740-modules-flat-same-name-ufcs-typing/a.sx

@@ -0,0 +1,6 @@
+#import "modules/std.sx";
+// a.sx authors `tag` returning a string; imported first → first-wins winner.
+// `show_a`'s `v.tag()` is the caller's OWN author (own == winner → existing UFCS
+// path, byte-for-byte unchanged): typed AND dispatched as a.tag (string).
+tag :: (x: s64) -> string { return "a-string"; }
+show_a :: () { v : s64 = 10; print("a: v.tag() = {}\n", v.tag()); }

examples/0740-modules-flat-same-name-ufcs-typing/b.sx

@@ -0,0 +1,7 @@
+#import "modules/std.sx";
+// b.sx authors its OWN `tag` returning s64. `show_b`'s `v.tag()` must be both
+// dispatched AND typed as b.tag (s64 = 110), not the first-wins winner from a.sx
+// (string). `print` types each arg from the call plan, so a mistype here boxes
+// the s64 as a string pointer → segfault before the fix.
+tag :: (x: s64) -> s64 { return x + 100; }
+show_b :: () { v : s64 = 10; print("b: v.tag() = {}\n", v.tag()); }