MongoDB sessions: no resurrection on save, legacy purge, key escaping; CI

[taplin]

Follow-ups to #14 from the adversarial review, plus the repo's first CI.

MongoDB backend

• Logout can't be undone by a racing save. save() now uses replaceOne(upsert: false). When nothing matched, it logs at debug level, since this is expected after a logout. Trade-off: a session whose create() insert failed, or that the TTL index already removed, is no longer re-created by the next save.

• Legacy documents. setup() deletes documents left by the archived Perfect-Session-MongoDB: string token, string data, created/updated/idle present, and no expiresAt. This driver can't resume them, and the TTL index never expires them. The purge isn't in clean(), so during a rolling deploy it won't keep deleting sessions that old instances are still writing. Unrelated documents with a token field are left alone (tested). The README has a migration section (en + zh_CN).

• Keys libmongoc won't store. Probed against MongoDB 8.3:

  • libmongoc rejects an empty key, which failed the whole write (only logged).
  • A key containing NUL was silently cut short.
  • $x and a.b are fine.

  Keys in data are now escaped at every depth: % becomes %25, NUL becomes %00, and "" becomes %. Other keys are stored unchanged, so data.<key> queries still work. Data that main's driver already stored with % in a key would read back differently, but no tagged release contains the MongoDB driver yet.

• Perfect-MongoDB floor raised to from: "4.0.2".

CI and tests

• New .github/workflows/ci.yml:
  • Linux (swift:6.4-resolute): every backend. SQLite runs through libsqlite3-dev, now that Build on Linux: add PerfectCSQLite system-library target Perfect-SQLite#23 builds on Linux. MongoDB, PostgreSQL, MySQL and Redis run against service containers.
  • macOS (macos-26): build plus the core and SQLite tests (no servers). It sets PKG_CONFIG_PATH for keg-only libpq/mysql-client.
• The SQLite, PostgreSQL, MySQL and Redis suites are now .serialized, because each test sets the connector's global settings. The review saw the SQLite suite fail or segfault intermittently on main. Separately, 2 of 6 SQLite tests failed in parallel on Linux; with .serialized, 12 release runs on macOS passed.
• The README (en + zh_CN) no longer calls SQLite macOS-only.

Verified

• The three new server tests (empty key, save after destroy, legacy purge) fail on main and pass here.
• Full suite on macOS, debug and release (-c release -Xswiftc -enable-testing), with MongoDB and SQLite.
• Linux, Swift 6.4 on Ubuntu 26.04 in a container: all 56 tests against MongoDB 8, PostgreSQL 17, MySQL 8.4 and Redis 7, plus SQLite.
• Not verified: this workflow hasn't run on GitHub before this PR, and I couldn't confirm the Swift version of the macos-26 runner's default Xcode.

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