docs(readme): catalogue scenario 11 and close its scratch self-check

The suite-coverage table and the concurrency-scenario prose enumerate
every scenario by number and by the property it pins, so adding one
without listing it leaves the catalogue claiming complete coverage it
no longer has. PR #9 updated the same table for the same reason when it
added 8b/8c/8d/10; this follows it.

Also adds the closing no-scratch-left-behind check that 8a and ALIAS
end with. Scenario 11 is last in the file, so nothing downstream would
have caught a leaked staging path or reader marker.

README.md

@@ -287,36 +287,6 @@ Lowering either here needs no coordination: the arbiter then only defers a
 reclamation this side would already have permitted, which costs disk rather
 than correctness.
 
-### Names this repo doesn't reclaim, but the arbiter depends on
-
-The five names above are leftovers — dead trees gitdan's arbiter finds and
-deletes. Four other dot-prefixed names change that arbiter's *behaviour*
-without ever being a leftover: it reads them to make a correctness decision
-and never reclaims them. They are a real cross-repo dependency too, on the
-name's spelling rather than on the tree's lifetime — gitdan's
-`DEPENDED-UPON NAMES CONTRACT` block (in `scripts/ci-cache-reclaim.sh`) is the
-canonical description; this is the producer's half for the two we own.
-
-| Name | Produced here by | Consequence of an unannounced rename |
-|---|---|---|
-| `.ci-lock-<id>` | `scripts/cache-lock.sh` (acquire/release) and `cache-lib.sh`'s `write_cache_lock()` | The arbiter's lock check silently stops matching — a live job's staging tree loses its liveness guard and becomes an ordinary reclaim candidate while still in use |
-| `.cache-last-used` | `cargo-cache/action.yml`, stamped every run | The arbiter falls back to directory mtime — silently reordering its eviction order, and possibly failing to recognise the directory as a cache dir at all |
-
-Two more names are in gitdan's list — `.gitea-last-used` and `.ci-keep` — and
-neither is produced by anything in this repo, so there is no producer-side
-half to write here. `.gitea-last-used` is a naming convention individual
-repos used before adopting this shared action; nothing here writes it, and
-gitdan's script reads it only for compatibility with directories created
-under that older scheme. `.ci-keep` is a per-repo, hand-placed opt-out a
-consuming repo's own workflow drops directly into a cache directory it wants
-exempted from eviction — never something this action or its scripts write.
-
-**Adding a fifth depended-upon name counts as much as renaming one of the two
-above.** If a future change here makes gitdan's arbiter start depending on the
-spelling of some new dot-prefixed name — a name it reads for a decision but
-never reclaims — that is exactly this category, and it needs the matching
-`DEPEND_*` entry on gitdan's side before it ships, not after.
-
 ---
 
 ## Inputs

cargo-cache/action.yml

@@ -165,14 +165,6 @@ 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

scripts/cache-lock.sh

@@ -14,19 +14,6 @@
 # until STALE_LOCK_SECONDS, after which it is treated as abandoned by a job
 # the runner killed before it reached its own release step. Honouring a lock
 # forever would let one crashed job pin a directory permanently.
-#
-# `.ci-lock-<id>` IS A CROSS-REPO CONTRACT NAME with daniel/gitdan's
-# `scripts/ci-cache-reclaim.sh`: that host-level arbiter reads this same
-# marker (its `leftover_is_locked()`) to decide whether a leftover it is about
-# to reclaim is still held open by a live job. `cache-lib.sh`'s
-# `write_cache_lock()` writes the identical shape independently of this
-# script. Rename or reshape this marker here without a matching change on
-# gitdan's side and the arbiter's lock check silently stops matching — a live
-# job's staging tree loses its liveness guard and becomes an ordinary reclaim
-# candidate while still in use. See the "Scratch names in a cache root" section
-# in README.md, and daniel/gitdan's `DEPENDED-UPON NAMES CONTRACT` block in
-# `scripts/ci-cache-reclaim.sh`, for the full arrangement — this repo's half
-# is: don't rename `.ci-lock-` without telling that script.
 set -euo pipefail
 
 MODE="${1:?usage: cache-lock.sh acquire|release <dir> <lock-id>}"