fix(hardlink): content freshness moved switches, it was not withdrawn

`unshare_mutable_paths`' comment recorded that upstream had stopped
rewriting `dep-<target>` in place, on a measurement taken against
1.100.0-nightly. It had not. Two unrelated cargo changes landed within
four days of each other and between them moved the switch that turns the
behaviour on and the path it writes to:

  - cargo PR #17382 (2026-08-22) demoted `-Z checksum-freshness` to a
    gate and gave `build.fingerprint` the choice, defaulting to `mtime`.
    Setting only the gate is accepted and does nothing, which is exactly
    the result that was read as a withdrawal.
  - build-dir layout v2 (cargo PR #17354, stable 1.100.0 on 2026-11-12,
    nightly default since 1.99) moved the file from
    `.fingerprint/<unit>/dep-*` to `build/<pkg>/<hash>/fingerprint/dep-*`.

Measured 2026-08-26 on 1.100.0-nightly (e8cb624d5): same toolchain, same
clone procedure, one env var apart — with the gate alone a `cp -al` clone
mutates only the build/ and *.d families; add
`CARGO_BUILD_FINGERPRINT=content` and the source's dep-info file is
mutated through the shared inode again. The hazard is intact.

So the suite now exports both switches, and its strongest scenario runs
again on current nightlies — verified passing against both layouts. Its
control note learned the v2 path too: it looked for the v1 path only, and
so printed "does NOT rewrite ... in place" three lines beneath a listing
that showed the rewrite.

Every claim these comments make is now dated and cited, because the
defect being fixed is a comment that cited one measurement and silently
stopped reproducing.

Layout v2 also drags the artifacts under `build/`, which collapses this
function's real-copy set from 21.3% of a target dir to 99.998%. That is a
live cost, not a correctness problem, and it is filed as gitdan-actions#14
rather than fixed here.

Part of daniel/gitdan#62.

scripts/restore-mtimes.sh

@@ -64,7 +64,8 @@
 # two refs' fingerprints ever share a directory and this script never has to
 # arbitrate freshness across refs — only within one ref's own history, which
 # is exactly what it is built to do soundly. On a nightly toolchain,
-# CARGO_UNSTABLE_CHECKSUM_FRESHNESS is a complementary, stronger guarantee
+# CARGO_UNSTABLE_CHECKSUM_FRESHNESS plus CARGO_BUILD_FINGERPRINT=content (both,
+# since cargo PR #17382 on 2026-08-22) is a complementary, stronger guarantee
 # (content-addressed rather than mtime-based freshness); this script is not
 # made redundant by it, because directory-form `rerun-if-changed` build-script
 # watches are not covered by it and stable historical mtimes stay cheap