font-store: clarify the lock acquisition cleanup sweep limit

crates/font-store/README.md

@@ -141,8 +141,8 @@ let installed = store.installed_language_packs();
 Removal restores the original folder if the installed-record update fails. If restoration also
 fails, the error names a preserved backup under `language-packs/.removing/`. That recovery
 folder is outside the ordinary download staging sweep; cleanup does not delete the backup.
-Committed removals record their backup IDs in the same atomic record update. Each locked store
-operation attempts cleanup of at most four recorded backups, completing finite rounds through
+Committed removals record their backup IDs in the same atomic record update. On lock acquisition,
+the cleanup sweep attempts at most four previously recorded backups, completing finite rounds through
 the receipts within a live store so failed entries are revisited even as new receipts arrive.
 This bounds the number of
 deletion attempts, not the work to delete one backup. Recreating the store resets the cursor;
@@ -150,6 +150,8 @@ backups and receipts remain available for later retries, and the next lock need
 pending cleanup. Failed rollback backups have no receipt and remain available for recovery.
 Receipts stay until cleanup succeeds and a later record write prunes them; their IDs cannot be
 reused for a new removal while they remain recorded.
+An installed record rebuilt after corruption has no cleanup receipts; unreceipted backups are
+retained for manual recovery.
 
 **Unpacking is hostile-input safe:** pure-Rust zstd (`ruzstd`, MIT; every frame is read, and nothing but tar padding may follow the archive) and `tar` (MIT OR Apache-2.0); regular files and the `LICENCES` folder only (no links or devices). Only the contract's names are allowed: `pack.toml`, `spelling.aff`, `spelling.dic`, `hyphenation.pat` at the root and `LICENCES/<name>` (`[A-Za-z0-9._-]`, no leading or trailing dot, no Windows device names such as `CON` or `nul.txt`). Each file at most 64 MiB, 256 MiB and 256 entries per pack. `pack.toml` is required and must name the entry's locale; the files the index promises (`spelling`, `hyphenation`) must be there. Index entries need a lowercase BCP 47 locale (listed once, not a Windows device name), a sha256 and an `https://` URL; names, licences, versions, classes and the release lose control and bidi/format characters before they are stored or listed; an optional `source` must be an `https://` URL. At most 4096 zstd frames per archive. An unreadable `language-packs.json` is copied to `language-packs.json.bad` and rebuilt from the pack folders (each `pack.toml`) atomically in place, as is a missing one when pack folders are there, so no installed pack drops out of the list. Only an install or removal (under the store lock) rewrites it; listing reads the folders meanwhile and writes nothing.