feat(core): prefix-filtered AcceptChannelsAsync overload

[Kiryuumaru]

Summary

Unifies IStreamMultiplexer.AcceptChannelsAsync to a single signature with an optional channelIdPrefix:

IAsyncEnumerable<IReadChannel> AcceptChannelsAsync(string? channelIdPrefix = null, CancellationToken ct = default);

• channelIdPrefix == null (default) — yields every inbound channel not claimed by a specific AcceptChannel call. Same behavior as before.
• channelIdPrefix != null — yields only channels whose ID starts with the prefix. Matched channels are routed exclusively to that enumeration and are not yielded by the unfiltered overload.

Why

Overlay protocols (e.g. mesh routing) and host applications need to share a multiplexer without stealing each other's channels. The previous accept-and-discard pattern stole host-app channels whenever the overlay was iterating. This routes them at the registry layer instead.

Implementation

ChannelRegistry tracks prefix subscriptions and dispatches each inbound channel in EnqueueForAccept by:

1. Exact-ID pending accept (AcceptChannel(id))
2. Prefix subscription (first-match, ordinal)
3. Default queue (unfiltered AcceptChannelsAsync())

Breaking Change

Per project policy (unreleased, no compatibility constraint), callsites that passed CancellationToken positionally must now use named-argument syntax:

// before
mux.AcceptChannelsAsync(ct)

// after
mux.AcceptChannelsAsync(ct: ct)

All in-repo callsites (5 samples + 11 test files) updated.

Verification

• dotnet build NetConduit.slnx — 0 warnings, 0 errors across net8.0/net9.0/net10.0
• NetConduit.UnitTests — 333 passed / 1 skipped (pre-existing) / 0 failed
• NetConduit.Transport.Tcp.IntegrationTests — 3/3 passed
• Targeted AcceptChannel tests — 12/12 passed