Overhaul regression testing: e2e suite, fuzzing, CI, and bug fixes

Add a protocol-level end-to-end suite (in-process fully wired server on
an ephemeral port pair, driven by hotline.Client over TCP) covering
handshake, login, public and private chat, message board, threaded
news, file list/download/upload, account admin, disconnect
notification, and shutdown broadcast. The harness retries on a fresh
port pair when another process steals a probed port before
ListenAndServe binds it, and Server gains WithConnectionRateLimit so
tests can disable the per-IP connection throttle.

Add native fuzz tests for Transaction, Field, and flattened file
object decoding, and fix the bugs the new tests surfaced:

- Transaction.Write panicked on out-of-range attacker-controlled size
  fields, and transactionScanner's uint32 length addition could wrap
  and yield a truncated token. The information fork size declared in
  an untrusted fork header is now bounded too.
- Client keepalive read c.done unsynchronized while Disconnect
  replaces it under the mutex.
- The shared Agreement's Seek+ReadAll login path raced concurrent
  logins; the server now prefers an AgreementBytes() snapshot.

Fill unit-test gaps (main's config-copy helpers, file resume data,
ReloaderFunc, R2 error injection and env validation) and add a CI test
workflow (build/vet + race-enabled shuffled suite), fixed lint
workflow triggers with golangci-lint v2.6, and Makefile test/cover/
lint/fuzz targets.

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