feat: DTail fork — server/client feature development

Squashed development of the snonux/dtail fork's product code (internal/, cmd/)
since diverging from mimecast/dtail. Major areas:

- Read/output path: the former "turbo" channel-less path is now the single,
  default server-side read/output path for cat/grep/tail and MapReduce; the old
  channel-based path and its config/env toggles were removed.
- MapReduce: single aggregate implementation (server + serverless) fed directly
  by a processor pipeline, with input-exhausted finalization via the shutdown
  coordinator; high-concurrency and data-race fixes.
- Journal source reads (journal:unit.service) via journalctl, Linux-gated behind
  a journal-v1 capability.
- Auth-key fast reconnect: in-memory per-user public-key cache with TTL/max-keys,
  registered over an authenticated session (AUTHKEY), checked before
  authorized_keys.
- Interactive query reload (--interactive-query) with SESSION START/UPDATE
  generation boundaries and capability negotiation.
- Client-side deadlines: --timeout / --shutdownAfter as context deadlines;
  follow shutdown handling.
- Client logging: diagnostics-only daily log by default, opt-in payload tee via
  --log-payload.
- Numerous correctness fixes (buffer-pool double-recycle races, EOF-sentinel
  leaks, glob-expansion cap, TOCTOU in CSV parsing) with accompanying unit tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

2 files changed, 61 insertions, 0 deletions

diff --git a/internal/ctxutil/sleep.go b/internal/ctxutil/sleep.go
new file mode 100644
index 0000000..6965e8d
--- /dev/null
+++ b/internal/ctxutil/sleep.go
@@ -0,0 +1,29 @@
+package ctxutil
+
+import (
+	"context"
+	"time"
+)
+
+// Sleep waits for the delay or exits early when the context is canceled.
+// It returns true when the full delay elapsed.
+func Sleep(ctx context.Context, delay time.Duration) bool {
+	if delay <= 0 {
+		select {
+		case <-ctx.Done():
+			return false
+		default:
+			return true
+		}
+	}
+
+	timer := time.NewTimer(delay)
+	defer timer.Stop()
+
+	select {
+	case <-ctx.Done():
+		return false
+	case <-timer.C:
+		return true
+	}
+}
diff --git a/internal/ctxutil/sleep_test.go b/internal/ctxutil/sleep_test.go
new file mode 100644
index 0000000..f32b22c
--- /dev/null
+++ b/internal/ctxutil/sleep_test.go
@@ -0,0 +1,32 @@
+package ctxutil
+
+import (
+	"context"
+	"testing"
+	"time"
+)
+
+func TestSleepReturnsEarlyOnCancel(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	start := time.Now()
+	if Sleep(ctx, time.Second) {
+		t.Fatal("Sleep should stop when the context is canceled")
+	}
+	if elapsed := time.Since(start); elapsed > 50*time.Millisecond {
+		t.Fatalf("Sleep took too long to return after cancellation: %v", elapsed)
+	}
+}
+
+func TestSleepWaitsForDelay(t *testing.T) {
+	ctx := context.Background()
+
+	start := time.Now()
+	if !Sleep(ctx, 20*time.Millisecond) {
+		t.Fatal("Sleep should report success when the delay elapses")
+	}
+	if elapsed := time.Since(start); elapsed < 15*time.Millisecond {
+		t.Fatalf("Sleep returned too early: %v", elapsed)
+	}
+}