refactor(B7.2): move pack/variadic lowering to lower/pack.zig

Verbatim relocation of the 19-method pack cluster (pack element/value
projection, spread refs, pack indexing + diagnostics, pack-to-slice,
variadic arg lowering/packing, pack-slice materialisation, pack-fn
calls + monomorphisation, pack projection resolution, isPackFn/
isPackParam predicates) into src/ir/lower/pack.zig. 19 aliases on
Lowering keep all call sites unchanged.

ProjectionPosition and PackProjection move from lower/protocol.zig to
lower/pack.zig (their domain home, next to resolvePackProjection);
the Lowering type aliases are repointed, so external references
(lower.test.zig) are unchanged.

Method pub-flip: findReturnValueType.

Gate: zig build OK; zig build test 426/426; run_examples 541/0; zero
expected/ snapshot churn.

src/ir/lower/protocol.zig

@@ -43,7 +43,6 @@ const Function = inst_mod.Function;
 const Module = mod_mod.Module;
 const Builder = mod_mod.Builder;
 
-
 const lower = @import("../lower.zig");
 const Lowering = lower.Lowering;
 const Scope = lower.Scope;
@@ -204,13 +203,6 @@ pub fn instantiateParamProtocol(self: *Lowering, pd: *const ast.ProtocolDecl, ar
 // consults type-args, value position consults methods, with NO
 // cross-namespace fallback.
 
-pub const ProjectionPosition = enum { type_position, value_position };
-
-pub const PackProjection = union(enum) {
-    type_arg: u32, // index into the protocol's `type_params`
-    method: u32, // index into the protocol's `methods`
-    not_found, // `name` absent from the position-selected namespace
-};
 
 /// Find `name` in `protocol_name`'s type-arg namespace (`protocol($T,...)`).
 /// Returns the `type_params` index, or null (also for unknown protocols).