test(hardlink): report the mutation families, don't assert one of them

CI's nightly is 1.100.0-nightly (2026-08-25); the machine this suite was
written on had 1.96.0-nightly (2026-02-24). On the newer one the control's
`cp -al` clone mutates only the build/ and *.d families — upstream appears to
have stopped rewriting `.fingerprint/*/dep-*` in place under checksum
freshness — so the suite went red on the *absence* of a hazard.

That is the wrong shape for a gate. The control's job is to prove the hazard
exists at all, which a non-empty mutated set already does; naming one family
as mandatory makes the suite red whenever upstream stops doing something we
never wanted it to do, and red in the CONTROL, where a failure reads as "the
hazard is gone" rather than "upstream changed". It is now a note either way.

Nothing is given up. The fix scenario asserts the source is byte-identical
after a full rebuild in the clone, which covers every family the running Cargo
has, named or not — and the checksum-freshness scenario after it tests the
stale-reuse hazard directly. The dep-* line only ever documented which family
was in play.

`unshare_mutable_paths` keeps unsharing dep-* regardless, and its measurement
block now records both observations with their versions: 22 MB of a 6.9 GB
tree against a failure mode that is a wrong answer rather than a slow one.

scripts/hardlink-clone-selftest.sh

@@ -134,11 +134,26 @@ fi
 ok "raw cp -al clone mutates the source ($(printf '%s\n' "$ctl_mutated" | wc -l) paths)"
 printf '%s\n' "$ctl_mutated" | sed 's/^/       /'
 
+# Reported, not asserted, and the distinction is the point. The control's job
+# is to prove the hazard exists at all, which the non-empty set above already
+# does; this line records WHICH families a given Cargo exhibits.
+#
+# `.fingerprint/*/dep-*` is the worst of them — it carries the per-source
+# checksums, so mutating it through a shared inode turns a hardlink clone into
+# silent stale-artifact reuse rather than a slow build. It was measured on
+# cargo 1.9x nightly (see unshare_mutable_paths in cache-lib.sh) and is NOT
+# reproduced on 1.100.0-nightly (2026-08-25), where the control mutates only
+# the build/ and *.d families. Failing on its absence would mean this suite
+# goes red whenever upstream stops doing something we never wanted it to do —
+# and it would go red in the CONTROL, where a failure reads as "the hazard is
+# gone" rather than "upstream changed". Nothing is lost by reporting it: the
+# fix scenario below asserts the source is byte-identical after a full rebuild
+# in the clone, which covers every family this Cargo has, named or not.
 if [ "$CHECKSUM_MODE" = "on" ]; then
   if printf '%s' "$ctl_mutated" | grep -q '\.fingerprint/.*/dep-'; then
-    ok "control confirms the checksum-freshness dep-info file is among the mutated set"
+    echo "  note: this cargo DOES rewrite .fingerprint/*/dep-* in place under checksum freshness"
   else
-    fail "expected .fingerprint/*/dep-* in the control's mutated set under checksum freshness"
+    echo "  note: this cargo does NOT rewrite .fingerprint/*/dep-* in place; only the build/ and *.d families appear above"
   fi
 fi