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>

1 files changed, 10 insertions, 3 deletions

diff --git a/internal/config/config.go b/internal/config/config.go
index ee23829..0053c5b 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -3,6 +3,13 @@ package config
 import "github.com/mimecast/dtail/internal/source"
 
 const (
+	// DefaultMaxCommandFrameSize is the default maximum number of bytes that
+	// may be buffered between two ';' delimiters in the command protocol.
+	// Frames exceeding this limit cause the session to be closed immediately to
+	// prevent memory exhaustion. Individual server deployments may override this
+	// via ServerConfig.MaxCommandFrameSize.
+	DefaultMaxCommandFrameSize int = 1 << 20 // 1 MiB
+
 	// HealthUser is used for the health check
 	HealthUser string = "DTAIL-HEALTH"
 	// ScheduleUser is used for non-interactive scheduled mapreduce queries.
@@ -25,13 +32,13 @@ const (
 	DefaultHealthCheckLogger string = "none"
 )
 
-// Client holds a DTail client configuration.
+// Client holds DTail client configuration.
 var Client *ClientConfig
 
-// Server holds a DTail server configuration.
+// Server holds DTail server configuration.
 var Server *ServerConfig
 
-// Common holds common configs of both both, client and server.
+// Common holds configuration common to both client and server.
 var Common *CommonConfig
 
 // Setup the DTail configuration.