docs(cache): write down the producer half of the depended-upon names contract

daniel/gitdan's ci-cache-reclaim.sh hard-codes four dot-prefixed names it
reads to make correctness decisions but never reclaims as leftovers —
.ci-lock-*, .cache-last-used, .gitea-last-used, .ci-keep — none of which
carried a matching note here (gitdan#32, the follow-up shape to #28/#30
that #7/PR #8 covered for the leftover names specifically).

Of the four, this repo actually produces two:

- .ci-lock-<id>, written by scripts/cache-lock.sh (acquire/release) and
  cache-lib.sh's write_cache_lock(). Documents the contract at
  cache-lock.sh's header, the site someone renaming the marker would
  most likely be editing.
- .cache-last-used, stamped every run by cargo-cache/action.yml. Documents
  the contract at the exact line that writes it.

The other two are read by gitdan's script but produced by nothing in this
repo: .gitea-last-used is a legacy naming convention individual repos
used before adopting the shared cargo-cache action (nothing here writes
it today), and .ci-keep is a per-repo, hand-placed opt-out any consuming
repo's own workflow may drop directly into a cache directory, with no
single owner. Both get a paragraph in README.md's new subsection
explaining why no producer-side counterpart exists for them, rather than
inventing an owner this repo doesn't have.

README.md's "Scratch names in a cache root are a cross-repo contract"
section gains a new subsection, "Names this repo doesn't reclaim, but
the arbiter depends on", covering all four and pointing at gitdan's
DEPENDED-UPON NAMES CONTRACT block as the canonical description.

No behaviour change: comments and docs only.

Ref: gitdan#32. Consumer-side counterpart: daniel/gitdan (this branch's
sibling PR).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MJHKJFJVUjnvVsemVBnvrd

cargo-cache/action.yml

@@ -165,6 +165,14 @@ runs:
     # touched unconditionally every run: a run that hits the cache for every
     # crate may write nothing at all inside the tree, which would make a
     # just-used directory look stale to the eviction pass.
+    #
+    # `.cache-last-used` IS A CROSS-REPO CONTRACT NAME with daniel/gitdan's
+    # `scripts/ci-cache-reclaim.sh`, which reads this exact marker for its own
+    # LRU ordering and to recognise a directory as a cache dir at all. Rename
+    # it here without a matching change there and that script silently falls
+    # back to directory mtime for both — see README.md's "Scratch names in a
+    # cache root" section and the `DEPENDED-UPON NAMES CONTRACT` block in
+    # gitdan's script.
     - shell: bash
       run: |
         set -euo pipefail