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).

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)