fix(selftest): make the two compiler-backed suites survive a CI runner

The new gate's first run went red on both suites that drive a real Cargo.
Neither failure was a defect in what they test; both were assumptions about
the machine, which had only ever been a dev box with a nightly installed and
no colour forcing. Fixed here rather than waived — turning a gate on is what
obliges fixing what it finds.

COLOUR. Every assertion in restore-mtimes-selftest.sh, and one in
hardlink-clone-selftest.sh, reads cargo's own words out of a build log
(`Compiling libdep`, `Fresh probe`). gitdan-ci's runner image forces colour, so
cargo wrote `Compiling\e[0m libdep` into the log and `grep -q "Compiling
libdep"` stopped matching. The suite then reported the opposite of what
happened — the failure printed the log, and the log plainly said `Compiling
libdep`. Both suites now pin CARGO_TERM_COLOR=never, which is the format their
assertions are written against.

Red-proven: with the export removed and CARGO_TERM_COLOR=always,
restore-mtimes-selftest reproduces the CI failure verbatim ("expected to find:
Compiling libdep"); with it, 14/14 pass under the same forced colour.

THE NIGHTLY PROBE ASKED THE WRONG QUESTION. `cargo +nightly -V` answers "did a
cargo proxy called with +nightly exit 0", which is not "will this build have
checksum freshness": `-V` short-circuits before `-Z` is validated at all. So
the probe said "on" on a runner where the flag was not in effect, and the
suite asserted the checksum-freshness mutation family against a build that
never had it — failing in the CONTROL, where a failure reads as "the hazard is
gone" rather than "the toolchain is wrong". It now probes the capability:
`cargo +nightly -Z checksum-freshness locate-project`, the narrowest command
that actually parses the flag. It rejects the stable channel and an unknown
flag name alike, needs no network and builds nothing.

Red-proven: the old channel probe, pointed at a cargo without the flag,
reproduces the CI failure exactly.

AND THE OFF PATH DID NOT WORK EITHER. The suite's header claimed that without
checksum freshness "the test still covers the build/ and *.d families". It did
not: the final scenario backdates the source to 2001 and asserts the rebuild
is not Fresh, which is checksum-freshness-only reasoning. Under Cargo's
ordinary mtime freshness a 2001 source IS older than the artifact and Fresh is
the correct answer, so the scenario asserted a bug. It is now gated on the
mode and skipped loudly, like the control's dep-* assertion already was: 5
assertions with a nightly, 3 without.

CI installs a nightly as well as stable (nightly first, so stable stays the
default) — that hazard is the one this whole scheme exists to close, and a CI
that skips it is checking the cheap half.

README.md

@@ -544,7 +544,10 @@ bash scripts/selftest.sh --fast   # fixture-only suites, no compiler
 
 Both run in CI — `.gitea/workflows/ci.yaml`, one job, on pushes to `main` and
 on PRs that were non-draft when the run was created. It installs shellcheck
-and a stable Rust toolchain and references no credentials; the scratch
+and both a stable and a nightly Rust toolchain (nightly for
+`-Z checksum-freshness`, without which `hardlink-clone-selftest.sh` skips the
+two assertions that cover the silent-stale-reuse hazard) and references no
+credentials; the scratch
 workspaces the compiler-backed suites build use path dependencies only, so
 nothing reaches crates.io. It runs the full suite rather than `--fast`,
 because the two compiler-backed suites are the ones that check this scheme
@@ -559,7 +562,7 @@ change here reaches all of them at once. That is what the gate is for.
 | suite | covers |
 |---|---|
 | `cache-root-selftest.sh` | that a lineage nests one level and nothing else moves: no lineage resolves byte-for-byte to the cache root, two lineages on one cache key get disjoint target dirs, seed/publish/prune all stay inside their own lineage, a PR layers over its own lineage's base snapshot — **and one rejection per lineage name a reader elsewhere would stop seeing**, plus the publish-side mismatch guard |
-| `hardlink-clone-selftest.sh` | that a build in a clone cannot mutate its source — with a control proving a raw `cp -al` does. Needs a real compiler. |
+| `hardlink-clone-selftest.sh` | that a build in a clone cannot mutate its source — with a control proving a raw `cp -al` does. Needs a real compiler, **and a nightly for two of its five assertions**: the checksum-freshness dep-info file is the mutation that turns a hardlink clone into silent stale-artifact reuse, and it only exists under `-Z checksum-freshness`. Without one those two are skipped, loudly. |
 | `seed-target-dir-selftest.sh` | seed-source preference, lock-file stripping, two jobs racing on one cache key, **and one scenario per check a hardlink clone is validated against**: a source rotated wholesale, a subtree silently lost from the walk, a copy that reports failure over a tree both other checks read as whole, and a source identity that resolved at neither end — plus a staging tree that could not be privately owned being discarded rather than published, and the publisher's log showing it waited on the consumer's own reader-lock marker before reclaiming a rotated snapshot |
 | `publish-snapshot-selftest.sh` | the atomic swap, that a live consumer survives a republish, and the publisher's side of the rotation race: deferred reclamation under a live reader, and its sweep once the reader is gone |
 | `prune-cache-selftest.sh` | liveness, protection, locking, eviction order, self-clear, **and that a cache a job claims *inside* the check-to-unlink window survives it** — against a real scratch `origin` |