feat: rename ALPN prefix from alknet/ to alk/ (v0.1.1)

- CHANNELS_ALPN: b"alknet/channels" → b"alk/channels"
- CallAdapter::alpn(): b"alknet/call" → b"alk/call"
- derive_alpn_from_op_name: alknet/ prefix → alk/ prefix
- All ALPN string literals in src/ and docs/ updated
- ADR-004 amended with prefix rename rationale
- AGENTS.md, README.md updated
- Version bumped to 0.1.1

Review: docs/reviews/003-alpn-prefix-rename.md

Verification:
- cargo test: 542 passed, 0 failed
- cargo clippy --all-targets -- -D warnings: clean
- cargo fmt --check: clean
- cargo doc --no-deps: clean

docs/architecture/decisions/038-channelconnection-bidistreamsource.md

@@ -172,9 +172,9 @@ that both paths are available and the handler crate chooses.
 ### Recursive composition
 
 A `ChannelBidiStreamSource` is a `BidiStreamSource`, and `Connection::
-from_source` wraps it. A handler that is itself `alknet/channels` can open a
+from_source` wraps it. A handler that is itself `alk/channels` can open a
 sub-channels connection on a data channel. This is recursive composition:
-`alknet/channels` inside `alknet/channels`. It is allowed (the `Connection`
+`alk/channels` inside `alk/channels`. It is allowed (the `Connection`
 abstraction permits it) but not a feature designed for — the primary use
 case is one level of multiplexing. Recursive composition is a natural
 consequence of the abstraction, not a goal.