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/ir.zig

@@ -6,6 +6,7 @@ pub const interp = @import("interp.zig");
 pub const lower = @import("lower.zig");
 pub const program_index = @import("program_index.zig");
 pub const type_resolver = @import("type_resolver.zig");
+pub const packs = @import("packs.zig");
 
 pub const TypeId = types.TypeId;
 pub const TypeInfo = types.TypeInfo;
@@ -35,6 +36,7 @@ pub const Lowering = lower.Lowering;
 pub const ProgramIndex = program_index.ProgramIndex;
 pub const TypeResolver = type_resolver.TypeResolver;
 pub const ResolveEnv = type_resolver.ResolveEnv;
+pub const PackResolver = packs.PackResolver;
 
 pub const compiler_hooks = @import("compiler_hooks.zig");
 pub const emit_llvm = @import("emit_llvm.zig");
@@ -55,6 +57,7 @@ pub const interp_tests = @import("interp.test.zig");
 pub const lower_tests = @import("lower.test.zig");
 pub const program_index_tests = @import("program_index.test.zig");
 pub const type_resolver_tests = @import("type_resolver.test.zig");
+pub const packs_tests = @import("packs.test.zig");
 pub const type_bridge_tests = @import("type_bridge.test.zig");
 pub const emit_llvm_tests = @import("emit_llvm.test.zig");
 pub const jni_descriptor_tests = @import("jni_descriptor.test.zig");