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.

1 files changed, 20 insertions, 10 deletions

diff --git a/hotline/server_blackbox_test.go b/hotline/server_blackbox_test.go
index 7033703..bfd9e9c 100644
--- a/hotline/server_blackbox_test.go
+++ b/hotline/server_blackbox_test.go
@@ -16,21 +16,31 @@ func NewTestLogger() *slog.Logger {
 	return slog.New(slog.NewTextHandler(os.Stdout, nil))
 }
 
-// assertTransferBytesEqual takes a string with a hexdump in the same format that `hexdump -C` produces and compares with
-// a hexdump for the bytes in got, after stripping the create/modify timestamps.
-// I don't love this, but as git does not  preserve file create/modify timestamps, we either need to fully mock the
-// filesystem interactions or work around in this way.
-// TODO: figure out a better solution
+// flatFileTimestampOffset is the byte offset of the information fork's CreateDate within a
+// flattened file object stream: FlatFileHeader (24) + INFO FlatFileForkHeader (16) + the info
+// fork's fixed fields up to CreateDate (Platform+TypeSignature+CreatorSignature+Flags+
+// PlatformFlags = 20, then RSVD = 32). CreateDate and ModifyDate are 8 bytes each and adjacent.
+const (
+	flatFileTimestampOffset = 24 + 16 + 20 + 32
+	flatFileTimestampLen    = 16 // CreateDate (8) + ModifyDate (8)
+)
+
+// assertTransferBytesEqual takes a string with a hexdump in the same format that `hexdump -C`
+// produces and compares with a hexdump for the bytes in got, after zeroing the info fork's
+// create/modify timestamps. Git does not preserve file create/modify times, so those bytes vary
+// between checkouts; the offset is derived structurally from the flattened file object layout
+// rather than hardcoded (see flatFileTimestampOffset).
 func assertTransferBytesEqual(t *testing.T, wantHexDump string, got []byte) bool {
 	if wantHexDump == "" {
 		return true
 	}
 
-	clean := slices.Concat(
-		got[:92],
-		make([]byte, 16),
-		got[108:],
-	)
+	clean := slices.Clone(got)
+	if len(clean) >= flatFileTimestampOffset+flatFileTimestampLen {
+		for i := flatFileTimestampOffset; i < flatFileTimestampOffset+flatFileTimestampLen; i++ {
+			clean[i] = 0
+		}
+	}
 	return assert.Equal(t, wantHexDump, hex.Dump(clean))
 }