refactor(ir): pack projections → PackResolver + retire the alias borrow (A2.3)

A2-merge gate: both parts in one commit, behavior-preserving (350/0).

Part 1 — retire the TypeTable.aliases borrow (build-enforced):
- type_bridge.zig: add `AliasMap` and thread it as an explicit param through
  every name-resolving fn (resolveAstType, bridgeType, resolveTypeName, the
  compound resolvers, resolveTupleLiteralAsType, resolveParameterizedType, the
  inline enum/struct/union + error resolvers). resolveTypeName now forwards the
  threaded map to TypeResolver.resolveNamed instead of reading table.aliases.
- lower.zig: all 31 resolveAstType callers pass
  &self.program_index.type_alias_map; drop the lowerRoot loan.
- types.zig: remove the now-unused TypeTable.aliases field.
- type_bridge.test.zig: alias test passes alias_map explicitly; other calls
  pass null.

Part 2 — pack projections get one owner + no .void failure sentinel:
- New packs.zig (PackResolver, a *Lowering facade): moves
  resolveClosure/Tuple/FunctionTypeWithBindings, packTypeElems, packTypeArgs,
  elementProtocolTypeArg out of Lowering. Call sites route through
  Lowering.packResolver(); barrel-wired in ir.zig.
- The missing-projection `orelse .void` in packTypeArgs now emits a diagnostic
  and fills the slot with .unresolved (the tripwire sentinel), never a real
  .void; OOM `catch return .void` in the moved fns became .unresolved too.
  Legitimate no-return-type `else .void` defaults are preserved.
- packs.test.zig: packTypeArgs bound/unbound/no-constraint/no-state cases +
  the missing-projection backstop (diagnostic + .unresolved slot).

src/ir/types.zig

@@ -332,14 +332,6 @@ pub const TypeTable = struct {
     slice_arena: std.heap.ArenaAllocator,
     /// Target pointer size in bytes (4 for wasm32, 8 for 64-bit targets).
     pointer_size: u8 = 8,
-    /// Borrowed pointer to `Lowering.program_index.type_alias_map`. When set,
-    /// `resolveTypeName` consults it before falling through to
-    /// the empty-struct-stub default — so a name like `ShaderHandle`
-    /// (defined `ShaderHandle :: u32`) resolves to `u32` rather than
-    /// being interned as a fresh empty struct. Pointer lifetime is
-    /// the owning Lowering's; consumers must clear it before the
-    /// Lowering is torn down.
-    aliases: ?*const std.StringHashMap(TypeId) = null,
 
     pub fn init(alloc: Allocator) TypeTable {
         var table = TypeTable{