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>

3 files changed, 28 insertions, 0 deletions

diff --git a/internal/protocol/capabilities.go b/internal/protocol/capabilities.go
new file mode 100644
index 0000000..e5412c7
--- /dev/null
+++ b/internal/protocol/capabilities.go
@@ -0,0 +1,12 @@
+package protocol
+
+const (
+	// HiddenCapabilitiesPrefix identifies server capability advertisements sent over the hidden control channel.
+	HiddenCapabilitiesPrefix = ".syn capabilities "
+
+	// CapabilityQueryUpdateV1 marks support for in-flight query replacement over an existing session.
+	CapabilityQueryUpdateV1 = "query-update-v1"
+
+	// CapabilityJournalV1 marks support for journal protocol operations over an existing session.
+	CapabilityJournalV1 = "journal-v1"
+)
diff --git a/internal/protocol/protocol.go b/internal/protocol/protocol.go
index d53d07e..13b7fb9 100644
--- a/internal/protocol/protocol.go
+++ b/internal/protocol/protocol.go
@@ -7,6 +7,12 @@ const (
 	MessageDelimiter byte = '¬'
 	// FieldDelimiter delimits messagefields.
 	FieldDelimiter string = "|"
+	// AggregateMessageID is the leading field of a mapreduce aggregate-data
+	// message on the wire (AGGREGATE|hostname|serialized-data). It is the
+	// single source of truth used by the server to tag such messages and by
+	// the mapr client to recognise them, so protocol acks like "AUTHKEY OK"
+	// (which merely start with the letter 'A') are not mistaken for data.
+	AggregateMessageID string = "AGGREGATE"
 	// CSVDelimiter delimits CSV file fields.kj:w
 	CSVDelimiter string = ","
 	// AggregateKVDelimiter delimits key-values of an aggregation message.
diff --git a/internal/protocol/session.go b/internal/protocol/session.go
new file mode 100644
index 0000000..ec6433c
--- /dev/null
+++ b/internal/protocol/session.go
@@ -0,0 +1,10 @@
+package protocol
+
+const (
+	// HiddenSessionStartOKPrefix acknowledges a successful SESSION START request.
+	HiddenSessionStartOKPrefix = ".syn session start ok"
+	// HiddenSessionUpdateOKPrefix acknowledges a successful SESSION UPDATE request.
+	HiddenSessionUpdateOKPrefix = ".syn session update ok"
+	// HiddenSessionErrorPrefix reports a rejected SESSION request.
+	HiddenSessionErrorPrefix = ".syn session err "
+)