docs(roadmap): mark selective receive (#4) done

Record what landed for recv_match/try_recv_match and update the
no-select gotcha: selective receive stayed per-channel rather than
introducing a cross-channel mailbox.

task.md

@@ -123,8 +123,30 @@ Shipped. What landed vs the plan:
   dead-pid link stops a non-trapper / messages a trapper; `unlink` prevents
   propagation.
 
-### 4. Selective receive (independent track)
+### 4. Selective receive (independent track) ✅ DONE (`03f3875`)
-Channels are strict FIFO MPSC; there's no pattern-matched `receive`.
+Shipped. What landed vs the plan:
+- `Receiver::recv_match(pred) -> Result<T, RecvError>` scans the queued
+  `VecDeque` front-to-back, removes+returns the first match, leaves the rest in
+  arrival order; parks and re-scans when nothing matches. `try_recv_match` (the
+  non-blocking variant, mirroring `try_recv`) rolled in same commit.
+- Wakeup turned out cheaper than feared: `Sender::send` *already* took
+  `parked_receiver` on every push, so "wake on ANY send" needed no send-side
+  change. The one real edit was relaxing `Sender::drop` to unpark the parked
+  receiver on the last-sender drop regardless of queue emptiness — a selective
+  receiver can park on a non-empty no-match queue and must wake to observe
+  closure. No-op for plain `recv` (only ever parks on an empty queue); stress
+  suite stays green.
+- `pred` is `Fn(&T) -> bool` (not `FnMut`) on purpose: it's re-run from scratch
+  on every scan, so a stateful predicate would re-count surprisingly. It runs
+  under the channel lock — keep it cheap/pure, don't re-enter the channel.
+- Close semantics: `recv_match` returns `Err(RecvError)` only when closed AND no
+  queued message matches; a match is still returned on a closed channel.
+  Non-matches are left for a later `recv`.
+- Tests (`tests/selective_recv.rs`): out-of-order match pulled first; non-matches
+  remain in order; park-on-non-empty then wake on a match; closed-with-only-
+  non-matches → Err; closed-but-match-present → match; `try_recv_match` states.
+
+  Original plan, for reference:
 - Add `Receiver::recv_match(pred) -> T`: scan the queued `VecDeque`, remove+return
   first match, leave the rest in order; park and re-scan on new arrivals.
 - This changes channel wakeup: a parked selective receiver must wake on ANY send
@@ -166,7 +188,10 @@ Channels are strict FIFO MPSC; there's no pattern-matched `receive`.
 - **No `select`, no unified per-process mailbox.** Why the supervisor uses the
   single `supervisor_channel` funnel rather than N monitor channels. trap_exit
   resolved this by giving each trapping actor a dedicated `Receiver<ExitSignal>`
-  inbox (see #3); selective-receive delivery still needs its own decision.
+  inbox (see #3); selective receive (#4) stayed *per-channel* (`recv_match`
+  scans one channel's queue) rather than introducing a cross-channel mailbox —
+  if selective receive ever needs to span the monitor/trap inboxes too, that
+  cross-channel merge is the still-unmade decision.
 - **Cooperative-only**: preemption and (future) cancellation both depend on the
   actor reaching `check!()`/yield/alloc/blocking points.
 - `run()` is single-thread (`Config::exact(1)`); tests rely on deterministic