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/expected/0740-modules-flat-same-name-ufcs-typing.exit

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

examples/expected/0740-modules-flat-same-name-ufcs-typing.stderr

@@ -0,0 +1 @@
+

examples/expected/0740-modules-flat-same-name-ufcs-typing.stdout

@@ -0,0 +1,2 @@
+a: v.tag() = a-string
+b: v.tag() = 110