docs(worker): correct tier from 2 to 3 and fix dependency tier refs

worker depends on launcher (now correctly Tier 2) and logz (Tier 1),
placing it at Tier 3. The previous docs cited launcher as Tier 1 and
logz as Tier 0, both of which were wrong.

docs/adr/ADR-003-channel-task-queue.md

@@ -0,0 +1,53 @@
+# ADR-003: Channel-Based Buffered Task Queue
+
+**Status:** Accepted
+**Date:** 2026-03-18
+
+## Context
+
+A worker pool requires a mechanism to hand off work from callers to goroutines.
+Common options include a mutex-protected slice, a ring buffer, or a Go channel.
+The pool must support multiple concurrent producers (callers of `Dispatch`) and
+multiple concurrent consumers (worker goroutines), while providing a simple
+backpressure signal when capacity is exhausted.
+
+## Decision
+
+The task queue is a buffered `chan Task` with capacity `Config.BufferSize` (env
+`WORKER_BUFFER_SIZE`, default 100). All worker goroutines receive from the same
+channel using `for task := range w.taskQueue`. Producers call `Dispatch` which
+uses a non-blocking `select` with a `default` branch:
+
+```go
+select {
+case w.taskQueue <- task:
+    return true
+default:
+    // queue full — log and return false
+    return false
+}
+```
+
+`Dispatch` returns `bool`: `true` if the task was enqueued, `false` if the queue
+was full. The caller decides what to do with a rejected task (retry, log, discard).
+
+Closing the channel in `OnStop` is the drain signal: `range` over a closed channel
+drains buffered items and then exits naturally, so no separate "stop" message is
+needed.
+
+## Consequences
+
+- The channel scheduler distributes tasks across all `PoolSize` goroutines without
+  any additional synchronisation code.
+- Backpressure is explicit: a full queue returns `false` rather than blocking the
+  caller or growing unboundedly. Callers that must not drop tasks should implement
+  retry logic at their layer.
+- Channel capacity is fixed at construction time. There is no dynamic resizing; if
+  workload consistently fills the buffer, `BufferSize` or `PoolSize` must be tuned
+  in config.
+- Closing the channel is a one-way signal: once `OnStop` closes it, `Dispatch` must
+  not be called again. This is safe in practice because `launcher` ensures `OnStop`
+  is only called after the application has stopped dispatching work, but there is no
+  runtime guard against misuse.
+- The `for range` pattern requires no sentinel values and is idiomatic Go for
+  fan-out worker pools.