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.

docs/test-system-v2.md

@@ -32,12 +32,12 @@ categories out of the ordinary developer target.
 | Fixed point and self-host | `test-bootstrap` |
 | Host linker/platform behavior | `test-platform` |
 
-The live declarative compiler corpus has 1,224 fixtures and 2,448 C/WW cells:
+The live declarative compiler corpus has 1,225 fixtures and 2,450 C/WW cells:
 314 expected rejections (290 shared and 24 stage-specific), 12 compile-only
-successes, 136 exit-zero programs, and 762 explicit-exit programs.
+successes, 136 exit-zero programs, and 763 explicit-exit programs.
 
-175 native C carriers remain. They are partitioned exactly once as five
-in-process units, 27 byte/artifact gates, seven bootstrap gates, one platform
+174 native C carriers remain. They are partitioned exactly once as five
+in-process units, 27 byte/artifact gates, six bootstrap gates, one platform
 gate, and 135 residual compiler, package-layout, ABI, diagnostic-observer,
 driver, linker, or FFI gates. Rows migrated to fixtures or native `@test`
 owners were removed from those carriers; there is no compatibility execution
@@ -59,7 +59,7 @@ wwstage-driver leg.
 | `test-library` | Library-owned `@test` behavior plus four direct standalone-source C/WW compilation checks |
 | `test-commit` | Unit + compiler + package + language + library behavior |
 | `test-byteid` | Compiler-output identity gates |
-| `test-bootstrap` | Fixed-point bootstrap plus the 950/990–995 native gates |
+| `test-bootstrap` | Fixed-point bootstrap plus the 950/991–995 native gates |
 | `test-platform` | Host-dependent dynamic-link gate |
 | `test-wwfixture` | Fixture CLI/process/protocol integration boundary |
 | `test-all` | Commit + byte-ID + bootstrap + platform + test-infrastructure checks |
@@ -196,7 +196,7 @@ parity is owned by the fixture corpus itself. Known cs/ww divergences are
 pinned in `DATABYTEID_DIVERGED` with the `989_lib_byteid` discipline: a
 pinned fixture must still build on both stages and still differ, so a
 compiler fix fails the gate demanding graduation rather than silently
-widening coverage. The full sweep compares 910 fixtures in about a minute
+widening coverage. The full sweep compares 911 fixtures in about a minute
 and is scratch-rooted under `out/`, not `/tmp`.
 
 Byte identity is an explicit proof gate. It is not a prerequisite of `test` or
@@ -204,7 +204,7 @@ Byte identity is an explicit proof gate. It is not a prerequisite of `test` or
 
 ## Bootstrap, subprocesses, and CSP
 
-Stage-2-through-stage-4 fixed-point proofs and the 950/990–995 self-host gates
+Stage-2-through-stage-4 fixed-point proofs and the 950/991–995 self-host gates
 are reachable through `test-bootstrap` and `test-all`, never through `test` or
 `test-commit`. Cold ordinary targets may still build their C- and WW-stage tool
 prerequisites once; they do not iterate those tools to a fixed point. The C