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/calls.zig

@@ -153,7 +153,7 @@ pub const CallResolver = struct {
                 if (fd.type_params.len > 0) {
                     return .{
                         .kind = .generic_fn,
-                        .return_type = self.l.inferGenericReturnType(fd, c),
+                        .return_type = self.l.genericResolver().inferGenericReturnType(fd, c),
                         .target = .{ .named = name },
                         .expands_defaults = defaultsFor(fd, c.args.len),
                     };