ffi M5.A.next.2b.fu1.A: mixed comptime+pack — lock in unresolved-tag miss

Follow-up #1 from step 2b: pack-fns that mix a non-pack
comptime param with the trailing pack (e.g. `tagged($tag: s32,
..$args)`). Today's `isPackFn` requires the pack to be the
ONLY comptime param; mixed shapes fall through to the inline
`lowerComptimeCall` path. That path adds non-string comptime
params to `comptime_param_nodes` for #insert substitution but
does NOT bind them as runtime locals, so the body's bare
`tag` reference hits "unresolved 'tag'" at the call site.

Next commit:
- Relax `isPackFn` to "exactly one trailing pack + any number
  of non-pack comptime params" so the mono path takes over.
- Fold comptime VALUES into the mangled name (`tagged(7, ...)`
  and `tagged(9, ...)` get distinct monos so each body sees
  its own comptime constants).
- Bind comptime args as both `comptime_param_nodes` (for
  #insert substitution) AND runtime locals (for bare-name
  references). String literals stay as string locals;
  int/bool/float literals become typed locals of the
  appropriate primitive type.

This is the load-bearing prerequisite for step 6 (stdlib
`print`/`format` refactor to `(\$fmt, ..\$args)`) — without
mixed-mode mono support, stdlib stays on the inline path
forever.

203/203 example tests + `zig build test` green (the lock-in
captures the wrong-shape diagnostic as the snapshot to flip).

examples/164-pack-mixed-comptime.sx

@@ -0,0 +1,29 @@
+// Variadic heterogeneous type packs — follow-up #1 (mixed
+// `$comptime + ..$args` pack-fn signatures).
+//
+// Today's `isPackFn` rejects pack-fns that mix any other
+// comptime param with the trailing pack — they fall through
+// to the inline `lowerComptimeCall` path. The inline path
+// doesn't bind non-string comptime params as runtime locals,
+// so a body that uses both `$tag` (s32) AND `..$args` fails
+// at the bare-name lookup of `tag`.
+//
+// Next commit relaxes `isPackFn` to accept "exactly one
+// trailing pack + any number of non-pack comptime params" and
+// `monomorphizePackFn` folds the comptime VALUES into the
+// mangled name (so distinct calls of `tagged(7, ...)` vs
+// `tagged(9, ...)` get distinct monos), then binds the
+// comptime values as both comptime substitutions and runtime
+// locals (for body code that references them by name).
+
+#import "modules/std.sx";
+
+tagged :: ($tag: s32, ..$args) -> s64 {
+    return tag * 100 + args.len;
+}
+
+main :: () -> s32 {
+    print("{}\n", tagged(7, 1, 2, 3));   // 7*100 + 3 = 703
+    print("{}\n", tagged(9));             // 9*100 + 0 = 900
+    return 0;
+}

tests/expected/164-pack-mixed-comptime.exit

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

tests/expected/164-pack-mixed-comptime.txt

@@ -0,0 +1 @@
+/Users/agra/projects/sx/examples/164-pack-mixed-comptime.sx:22:12: error: unresolved 'tag' (in /Users/agra/projects/sx/examples/164-pack-mixed-comptime.sx fn main)