fix(lower): resolved author drives variadic packing in bare call [0102c F1]

Attempt-2 fix for the F1 review finding. After `resolveBareCallee` picks a
shadowed same-name author's FuncId at a normal call site, the call path still
re-fetched the FIRST-WINS function AST by name to drive variadic argument
packing. When the resolved (shadow) author's variadic shape differs from the
first-wins author's, arguments were packed against the WRONG signature — a
fixed-arity shadow packed as if variadic, or a variadic shadow not packed at
all — producing IR with the wrong argument count (LLVM verification failure).

The `.func` arm now carries the resolved `*FnDecl` alongside its FuncId
(`BareCallee.func: ResolvedAuthor`), so `packVariadicCallArgs` reads THE
resolved author's signature. The rest of the arm already used the resolved
FuncId's IR function (ret/params/ctx/coercion), so the callee now has one
source of truth in the whole call lowering — no re-fetch by name after
resolution. Default-arg / closure / UFCS / comptime *sites* remain first-wins
(fix-0102d); `expandCallDefaults` runs before resolution and is a default site.

Regression: examples/0726-modules-flat-same-name-variadic — two flat file
imports each author `combine` and `pick` with OPPOSITE variadic shapes (a.sx
fixed `combine` / variadic `pick`; b.sx variadic `combine` / fixed `pick`).
Each module's bare call must pack against ITS OWN author. Fails on the pre-fix
re-lookup (LLVM "Incorrect number of arguments passed to called function" for
both `combine.1` and `pick.2`); passes after.

Gate: zig build, zig build test (400/400), bash tests/run_examples.sh
(463 passed) all green. Matrix 0722-0725/0727 unchanged; single-author / local
resolution byte-for-byte unchanged (the `.func` arm never runs for them).

examples/0726-modules-flat-same-name-variadic/a.sx

@@ -0,0 +1,11 @@
+// a.sx is the first-wins winner for both names. `combine` is FIXED arity;
+// `pick` is VARIADIC. `from_a_*` call them bare — a authors the winner, so
+// they resolve through the existing path and pack against a's own shapes.
+combine :: (x: s64, y: s64) -> s64 { return x + y; }
+pick :: (..xs: []s64) -> s64 {
+    result := 0;
+    for xs: (it) { result = result + it; }
+    result
+}
+from_a_combine :: () -> s64 { return combine(10, 20); }
+from_a_pick :: () -> s64 { return pick(1, 2, 3); }

examples/0726-modules-flat-same-name-variadic/b.sx

@@ -0,0 +1,12 @@
+// b.sx is the SHADOW author for both names, with the OPPOSITE shapes:
+// `combine` is VARIADIC, `pick` is FIXED. Each `from_b_*` bare call must pack
+// against b's OWN author's signature (the F1 fix) — combine sums its variadic
+// pack, pick subtracts its two fixed args.
+combine :: (..xs: []s64) -> s64 {
+    result := 0;
+    for xs: (it) { result = result + it; }
+    result
+}
+pick :: (a: s64, b: s64) -> s64 { return b - a; }
+from_b_combine :: () -> s64 { return combine(1, 2, 3, 4); }
+from_b_pick :: () -> s64 { return pick(2, 7); }