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, 12 insertions, 12 deletions

diff --git a/hotline/client_test.go b/hotline/client_test.go
index 23b6155..fb5a3a9 100644
--- a/hotline/client_test.go
+++ b/hotline/client_test.go
@@ -21,8 +21,8 @@ func newTestClient() *Client {
 func TestClient_Handshake(t *testing.T) {
 	t.Run("successful handshake", func(t *testing.T) {
 		clientConn, serverConn := net.Pipe()
-		defer clientConn.Close()
-		defer serverConn.Close()
+		defer func() { _ = clientConn.Close() }()
+		defer func() { _ = serverConn.Close() }()
 
 		c := newTestClient()
 		c.Connection = clientConn
@@ -41,8 +41,8 @@ func TestClient_Handshake(t *testing.T) {
 
 	t.Run("server returns error response", func(t *testing.T) {
 		clientConn, serverConn := net.Pipe()
-		defer clientConn.Close()
-		defer serverConn.Close()
+		defer func() { _ = clientConn.Close() }()
+		defer func() { _ = serverConn.Close() }()
 
 		c := newTestClient()
 		c.Connection = clientConn
@@ -61,7 +61,7 @@ func TestClient_Handshake(t *testing.T) {
 
 	t.Run("connection closed during read", func(t *testing.T) {
 		clientConn, serverConn := net.Pipe()
-		defer clientConn.Close()
+		defer func() { _ = clientConn.Close() }()
 
 		c := newTestClient()
 		c.Connection = clientConn
@@ -69,7 +69,7 @@ func TestClient_Handshake(t *testing.T) {
 		go func() {
 			buf := make([]byte, 12)
 			_, _ = io.ReadFull(serverConn, buf)
-			serverConn.Close()
+			_ = serverConn.Close()
 		}()
 
 		err := c.Handshake()
@@ -81,8 +81,8 @@ func TestClient_Handshake(t *testing.T) {
 func TestClient_Send(t *testing.T) {
 	t.Run("sends non-reply transaction and tracks it", func(t *testing.T) {
 		clientConn, serverConn := net.Pipe()
-		defer clientConn.Close()
-		defer serverConn.Close()
+		defer func() { _ = clientConn.Close() }()
+		defer func() { _ = serverConn.Close() }()
 
 		c := newTestClient()
 		c.Connection = clientConn
@@ -104,8 +104,8 @@ func TestClient_Send(t *testing.T) {
 
 	t.Run("reply transactions are not tracked", func(t *testing.T) {
 		clientConn, serverConn := net.Pipe()
-		defer clientConn.Close()
-		defer serverConn.Close()
+		defer func() { _ = clientConn.Close() }()
+		defer func() { _ = serverConn.Close() }()
 
 		c := newTestClient()
 		c.Connection = clientConn
@@ -195,7 +195,7 @@ func TestClient_HandleTransaction(t *testing.T) {
 func TestClient_Disconnect(t *testing.T) {
 	t.Run("closes connection and done channel", func(t *testing.T) {
 		clientConn, serverConn := net.Pipe()
-		defer serverConn.Close()
+		defer func() { _ = serverConn.Close() }()
 
 		c := newTestClient()
 		c.Connection = clientConn
@@ -220,7 +220,7 @@ func TestClient_Disconnect(t *testing.T) {
 
 	t.Run("handles nil done channel", func(t *testing.T) {
 		clientConn, serverConn := net.Pipe()
-		defer serverConn.Close()
+		defer func() { _ = serverConn.Close() }()
 
 		c := newTestClient()
 		c.Connection = clientConn