refactor(ir): extract protocol/impl lookup into protocols.zig (A4.2 step 2)

Move the pure protocol/impl conformance lookups into one module,
src/ir/protocols.zig, behind a *Lowering facade (ProtocolResolver), mirroring
GenericResolver / CallResolver. Per PLAN-ARCH A4.2 ("move pure lookup first;
keep emission in Lowering"), this increment moves only the read-only queries:
- getProtocolInfo (is a type a registered protocol + its method table),
- hasImplPlain (have the (protocol, type) thunks been materialized),
- packArgConformsTo (impl-declaration-level conformance for ..xs: P).

Registration (registerProtocolDecl / registerImplBlock / registerParamImpl) and
all IR emission (createProtocolThunk / buildProtocolValue / tryUserConversion /
getOrCreateThunks) stay in Lowering for the later increments. The state maps
(protocol_thunk_map / param_impl_map on Lowering, protocol_decl_map /
protocol_ast_map in ProgramIndex) stay put; the facade reads them via self.l.* —
no map migration.

Lowering keeps getProtocolInfo as a thin pub wrapper (~9 callers incl.
calls.zig); hasImplPlain + packArgConformsTo are deleted (no fallback), their 3
call sites (computeHasImpl x2, the pack-conformance check x1) routed through
self.protocolResolver(). formatTypeName widened to pub (the lookups use it);
protocolResolver() accessor added.

protocols.test.zig (wired into the barrel) drives ProtocolResolver directly:
getProtocolInfo (registered vs builtin/plain-struct + wrapper delegation),
hasImplPlain (thunk-map materialization), packArgConformsTo (non-parameterised
requires <ty>.<m> in fn_ast_map; trivially-true for an erased protocol value;
false for unknown protocol).

zig build, zig build test, tests/run_examples.sh (357/0) all green — no .ir
snapshot churn; the 0410/0411/0412 rejection anchors still pass.

src/ir/ir.zig

@@ -10,6 +10,7 @@ pub const packs = @import("packs.zig");
 pub const expr_typer = @import("expr_typer.zig");
 pub const calls = @import("calls.zig");
 pub const generics = @import("generics.zig");
+pub const protocols = @import("protocols.zig");
 pub const semantic_diagnostics = @import("semantic_diagnostics.zig");
 
 pub const TypeId = types.TypeId;
@@ -45,6 +46,7 @@ pub const ExprTyper = expr_typer.ExprTyper;
 pub const CallResolver = calls.CallResolver;
 pub const CallPlan = calls.CallPlan;
 pub const GenericResolver = generics.GenericResolver;
+pub const ProtocolResolver = protocols.ProtocolResolver;
 
 pub const compiler_hooks = @import("compiler_hooks.zig");
 pub const emit_llvm = @import("emit_llvm.zig");
@@ -69,6 +71,7 @@ pub const packs_tests = @import("packs.test.zig");
 pub const expr_typer_tests = @import("expr_typer.test.zig");
 pub const calls_tests = @import("calls.test.zig");
 pub const generics_tests = @import("generics.test.zig");
+pub const protocols_tests = @import("protocols.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");