test/wcc: retire 990_selfhost; its live assertions move to their owners

Every probe's assertion is owned by a current gate: the compile and
link probes by make all and the bootstrap fixed point; build/run and
cs/ww byte identity by the fixture corpus, test-data-byteid, and
989_lib_byteid; wwstage driver and toolchain parity by 993/995;
checker-diagnostic parity by the corpus' both-stage //ww:error rows.
The wwdump -t/-a dump-parity probes gated the frontend port's
convergence, which the compiler-output identity gates now own end to
end; carrier ran green at retirement.

What was still uniquely alive migrates: smoke.ww becomes corpus
fixture selfhost_smoke (upgraded from a cstage-only build to both
frontends, byte-identical, exit 42 on both toolchains; corpus pins
move to 1,225/763/2,450 with the new identity hash in the same
commit), and sym_link.ww's scope/sym behavior rows become in-language
lib/ww/syntax/symtest.ww under LIBRARY_TESTS. uses.ww (parser-stub-era
-a fixture) and the already-orphaned tagged_ptr_ret.ww/trypromote.ww
retire with the probe corpus. Bootstrap native gates drop to six;
frontend numeric-sync comments now cite the rule-6 mirror instead of
the retired diff probe.

internal/wwfixture/types.ww

@@ -1,13 +1,13 @@
 package wwfixture;
 
 def protocolversion: i32 = 1;
-def corpuscount: i32 = 1224;
+def corpuscount: i32 = 1225;
 def errorcount: i32 = 314;
 def compilecount: i32 = 12;
 def runcount: i32 = 136;
-def runexitcount: i32 = 762;
-def nativecount: i32 = 2448;
-def corpushash: str = "8b522e42cbce8b389e33e917229dede8045d85f1af0c54630cfef83c4ef242be";
+def runexitcount: i32 = 763;
+def nativecount: i32 = 2450;
+def corpushash: str = "a2e47b2967d51a517d39598caf69c2d23618ba488594a0b7e33fded27cc62f93";
 
 type directive = enum i32 {
 	ERROR = 0,