docs: qualify the 256-channel figure — default policy knob, not a wire limit

channel_id is u32 on the wire (~4B ids, monotonic counter, ADR-040);
256 is the default per-connection max_channels (memory bound, 256x1MiB
reassembly buffers) and the default per-identity DoS cap (ADR-041) —
both configurable without a wire change. AGENTS.md convention 10 and
the OQ-SK-03 composed-egress note now carry the qualifier.

docs/research/phase-0.md

@@ -529,8 +529,10 @@ what gets each datagram to `addr`?
    handles* — because alktunnels' udp substrate is **connected**
    (`connect_udp(target)`: one tunnel, one fixed target) while
    ASSOCIATE names a different destination per datagram. Cost: one
-   open per destination (bounded by the 256-channel cap; fine for
-   DNS + browsing; the per-destination latency is a POC #2
+   open per destination (the per-connection/per-identity channel
+   defaults are 256 — ADR-040/041 policy knobs, configurable without a
+   wire change; the `u32` wire space is ~4 billion — so fine for DNS +
+   browsing, and the per-destination open latency is a POC #2
    measurement). Benefit: **per-target ACL for free** — the ACL
    governing the udp-tunnel resource governs egress per destination
    (dissolving most of OQ-SK-05 for the composed path), and hub