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, 8 insertions, 5 deletions

diff --git a/internal/clients/handlers/healthhandler.go b/internal/clients/handlers/healthhandler.go
index 47b594e..763ba88 100644
--- a/internal/clients/handlers/healthhandler.go
+++ b/internal/clients/handlers/healthhandler.go
@@ -19,11 +19,14 @@ func NewHealthHandler(server string) *HealthHandler {
 	dlog.Client.Debug(server, "Creating new health handler")
 	return &HealthHandler{
 		baseHandler: baseHandler{
-			server:       server,
-			shellStarted: false,
-			commands:     make(chan string),
-			status:       2, // Assume CRITICAL status by default.
-			done:         internal.NewDone(),
+			server:         server,
+			shellStarted:   false,
+			commands:       make(chan string),
+			status:         2, // Assume CRITICAL status by default.
+			done:           internal.NewDone(),
+			capabilities:   make(map[string]struct{}),
+			capabilitiesCh: make(chan struct{}),
+			sessionAcks:    make(chan SessionAck, 4),
 		},
 	}
 }