fix(lower): bare-call resolver skips non-plain authors before ambiguity gate [0102c F3]

resolveBareCallee's flat-collect branch counted ALL same-name authors —
including #foreign / generic / builtin / #compiler — before the
isPlainFreeFn filter, so two flat-imported modules each #foreign-ing the
same libc symbol under one sx name returned `.ambiguous` and errored,
instead of falling to `.none` and the existing first-wins foreign path
(master behavior). Filter authors to plain free functions DURING
collection, before the count/ambiguity determination: a non-plain
collision now yields 0 reroutable authors -> `.none`; genuine plain-fn
collisions still yield >= 2 -> `.ambiguous` (0724 unchanged). The
now-redundant single-author isPlainFreeFn check is dropped.

Regression: examples/0729-modules-flat-same-name-foreign — two flat FILE
imports each #foreign the same libc "abs" under name `absval`; a bare
call resolves first-wins and runs (exit 0). Fails-before on this branch
(ambiguity error), passes-after.

examples/expected/0729-modules-flat-same-name-foreign.exit

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

examples/expected/0729-modules-flat-same-name-foreign.stderr

@@ -0,0 +1 @@
+

examples/expected/0729-modules-flat-same-name-foreign.stdout

@@ -0,0 +1 @@
+absval = 7