lang 2.4: bind protocol-constrained packs (per-shape mono, concrete elements)

`..xs: Protocol` now binds like the comptime `..$args` pack instead of
falling through to a runtime `[]Protocol` slice: each call site
monomorphizes with the concrete per-position arg types, and `xs[i]` is the
concrete element via AST substitution (Decision 1 — a pack is a comptime
mechanism, no runtime pack value). So `xs[i]`'s own fields/methods dispatch
statically and elements may be heterogeneous, while `xs.len` is a comptime
constant.

Mechanism: one `isPackParam(p) = is_variadic and (is_comptime or is_pack)`
predicate replaces the four `is_variadic and is_comptime` pack-detection
sites (call-arg split, mangle, arg lowering, monomorphizePackFn), and the
early call dispatch routes any `isPackFn` call to `lowerPackFnCall` before
the `hasComptimeParams` gate (which is false for a protocol pack).

examples/191-protocol-pack.sx exercises N=0, N=2, concrete field access, and
a heterogeneous IntBox+StrBox pack. Conformance checking and projection
(`xs.T` / `xs.value`) are the remaining 2.4 work.

tests/expected/191-protocol-pack.exit

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

tests/expected/191-protocol-pack.txt

@@ -0,0 +1,4 @@
+count0=0
+count2=2
+sum=49
+describe len=2 first=42 second=cool