fix(lower): route remaining bare-name sites through resolver + close 0102 [0102d]

Final 0102 sub-step. fix-0102c landed resolveBareCallee and routed the
primary call path + parameter target typing through it, leaving four other
bare-name consumer sites on the old first-wins path. Route the SAME resolver
through all four, gated exactly as the call path (plain top-level identifier,
no scope-mangle / UFCS alias / local shadow; act on .func / .ambiguous, fall
through on .none so single-author / local / std / qualified / foreign-single
resolution is byte-for-byte unchanged):

1. Default-argument expansion (expandCallDefaults): omitted trailing args
   fill from the RESOLVED author's defaults, not the winner's.
2. Function-value conversion (closure(fn) and the bare-fn-as-value func_ref /
   fn-ptr / closure-coercion path): captures the resolved author's FuncId.
3. Free-function UFCS (recv.fn() -> fn(recv, ...)): dispatches the resolved
   author for the receiver's source.
4. Comptime #run of a bare call: lowerMainAndComptime now sets
   current_source_file per decl, so a `NAME :: #run f()` in an imported
   module resolves f from THAT module's flat imports (own-author wins) instead
   of the main file's perspective (which made it spuriously ambiguous).

Regression tests: examples/0730-0734 (default-arg, closure+fn-value, UFCS,
comptime #run, UFCS-ambiguity), each fails on pre-fix code and passes after.
issues/0102-flat-import-same-signature-collision.md written RESOLVED with the
4-sub-step root cause and regression-test paths.

examples/0730-modules-flat-same-name-default-arg/a.sx

@@ -0,0 +1,5 @@
+// a.sx authors `cfg` defaulting to 10. Imported first, so it is the first-wins
+// merge winner. `from_a` calls `cfg()` with the arg omitted — own == winner →
+// existing default-expansion path, byte-for-byte unchanged.
+cfg    :: (n: s64 = 10) -> s64 { return n; }
+from_a :: () -> s64 { return cfg(); }

examples/0730-modules-flat-same-name-default-arg/b.sx

@@ -0,0 +1,5 @@
+// b.sx authors its OWN `cfg` defaulting to 20. `from_b`'s `cfg()` omits the
+// arg; the omitted trailing default must come from b.sx's author (20), not the
+// first-wins winner from a.sx (10).
+cfg    :: (n: s64 = 20) -> s64 { return n; }
+from_b :: () -> s64 { return cfg(); }