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/program_index.zig

@@ -74,7 +74,8 @@ pub const ProgramIndex = struct {
     foreign_class_map: std.StringHashMap(*const ast.ForeignClassDecl) = std.StringHashMap(*const ast.ForeignClassDecl).init(std.heap.page_allocator),
     /// `#run` global name → GlobalId.
     global_names: std.StringHashMap(GlobalInfo),
-    /// Type alias name → target TypeId. Loaned to `TypeTable.aliases`.
+    /// Type alias name → target TypeId. The single-source alias table; passed
+    /// explicitly to `TypeResolver` / `type_bridge` resolution (no borrow).
     type_alias_map: std.StringHashMap(TypeId) = std.StringHashMap(TypeId).init(std.heap.page_allocator),
     /// Generic struct name → template.
     struct_template_map: std.StringHashMap(StructTemplate) = std.StringHashMap(StructTemplate).init(std.heap.page_allocator),