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, 34 insertions, 0 deletions

diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
new file mode 100644
index 0000000..3d4e155
--- /dev/null
+++ b/.github/workflows/test.yml
@@ -0,0 +1,34 @@
+name: test
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+      - name: Build
+        run: go build ./...
+      - name: Vet
+        run: go vet ./...
+      - name: Test
+        run: go test ./... -race -shuffle=on -coverprofile=coverage.out -covermode=atomic
+      - name: Coverage summary
+        run: |
+          echo '### Coverage' >> "$GITHUB_STEP_SUMMARY"
+          echo '```' >> "$GITHUB_STEP_SUMMARY"
+          go tool cover -func=coverage.out | tail -1 >> "$GITHUB_STEP_SUMMARY"
+          echo '```' >> "$GITHUB_STEP_SUMMARY"
+      - uses: actions/upload-artifact@v4
+        with:
+          name: coverage
+          path: coverage.out