refactor(ir): extract GenericResolver (generics.zig) for substitution + mono keys (A4.1 step 2)

Generic substitution and monomorphization-key construction now live in one
module, src/ir/generics.zig, behind a *Lowering facade (GenericResolver),
mirroring CallResolver / ExprTyper. Moved verbatim:
- mangleTypeName + mangleParamList (the mono-key fragment builder),
- mangleGenericName (generic mono key), appendComptimeValueMangle (comptime-value
  fragment),
- buildTypeBindings (call-site type-param inference), inferGenericReturnType
  (generic return resolution).

inferGenericReturnType now uses a scoped TypeBindingScope (enter/exit with defer)
instead of a manual type_bindings save/restore — the PLAN-ARCH A4.1 "scoped
substitution env" shape; a generics.test.zig assertion confirms the prior
bindings are restored (the issue-0048/0050 leak class, for this field).

Lowering keeps a thin pub mangleTypeName wrapper delegating to
genericResolver().mangleTypeName, because ~30 cross-cutting callers (impl-map
keys, conversion keys, shape keys) reach it well beyond generics. mangleParamList
(sole caller was mangleTypeName) moved fully. The other 4 originals are deleted
(no fallback); their 6 call sites now go through self.genericResolver()
(calls.zig via self.l.genericResolver()).

matchTypeParam / extractTypeParam / isTypeParamDecl widened to pub (the moved
substitution logic calls them); genericResolver() accessor added. The 2
mangleTypeName / inferGenericReturnType unit tests moved from lower.test.zig to
generics.test.zig (driving GenericResolver directly) and wired into the barrel.

monomorphizeFunction / monomorphizePackFn intentionally stay in lower.zig (they
save/restore three fields across nested mono and call emission helpers) — a
heavier scoped-env adoption deferred to an optional sub-step 3.

zig build, zig build test, and tests/run_examples.sh (357/0) all green — no .ir
snapshot churn, confirming the move preserved mono-key/substitution output.

src/ir/ir.zig

@@ -9,6 +9,7 @@ pub const type_resolver = @import("type_resolver.zig");
 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 semantic_diagnostics = @import("semantic_diagnostics.zig");
 
 pub const TypeId = types.TypeId;
@@ -43,6 +44,7 @@ pub const PackResolver = packs.PackResolver;
 pub const ExprTyper = expr_typer.ExprTyper;
 pub const CallResolver = calls.CallResolver;
 pub const CallPlan = calls.CallPlan;
+pub const GenericResolver = generics.GenericResolver;
 
 pub const compiler_hooks = @import("compiler_hooks.zig");
 pub const emit_llvm = @import("emit_llvm.zig");
@@ -66,6 +68,7 @@ pub const type_resolver_tests = @import("type_resolver.test.zig");
 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 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");