test: migrate Fam12 opaque value tests to @test row-tables (#5-C1)

fold-2 chunk C1 (drew's Fam8-13 plan): 960_opaque_decl_run.c and
962_opaque_assign_cast_run.c were value-row C drivers. Migrate their 3+3
cases to test/lang/opaque_decl_test.ww and opaque_assign_cast_test.ww as
@test row-tables. The test-lang byte-id (LANGBYTEID) gate gives cs==ww
automatically -- 960 was cstage-only-run before, so this strengthens it.
Both byte-id clean, no cs!=ww carve. Retire the 2 C drivers (LANGBYTEID floor
57->59) and repoint a dead comment ref in lib/sort/sort.ww. 961_opaque_guards
(reject/guards) stays in C -- fold-3 territory, not in the Fam12 fold-2
worklist.

lib/sort/sort.ww

@@ -19,7 +19,7 @@
 //   • Hare walks bytes through `in: *[*]u8` + `&ba[i*sz]`; ww has no
 //     unbounded-array `[*]`, so the base is `*u8` reinterpreted to
 //     `uintptr` and elements are `base + i*sz` (the stride idiom proven
-//     by test/wcc/962_opaque_assign_cast_run.c).
+//     by test/lang/opaque_assign_cast_test.ww).
 //   • `len()` yields i32 in ww (slice length is i32 today), so the
 //     count is cast `: size`; Hare's len() is already size.
 //   • ww `for` is single-condition only (no `for (init; cond; post)`),