refactor(core): add Connection::from_source public constructor (ADR-070 gap)

The BidiStreamSource trait (ADR-070) made Connection hold Box<dyn
BidiStreamSource> so downstream crates can add connection shapes without
editing core — but the public constructor for that path was missing.
The source field is private; the channels crate (or any future crate)
had no way to build a Connection from its own BidiStreamSource impl.
The trait was unusable from outside core.

Add Connection::from_source(source: impl BidiStreamSource, alpn: Vec<u8>)
-> Self: the extension point. Takes impl BidiStreamSource (not Box<dyn>)
to match the ergonomic style of from_stream / from_bidi; boxes the
source internally. No feature gate (always available, like from_stream).
Initializes alpn and identity: OnceLock::new() the same as the other
constructors.

Test: a custom RecordingSource BidiStreamSource impl (not a built-in)
constructed via from_source, verifying remote_alpn / remote_addr /
accept_bi (round-trips real bytes via tokio::io::duplex) / open_bi
(StreamClosed) / close (records code+reason) all delegate to the custom
impl.

Verified: cargo test + clippy clean across default, --no-default-features,
--features iroh, --all-features; cargo check -p alknet-call succeeds
unchanged; cargo fmt --check passes.

tasks/core/connection-from-source-constructor.md

@@ -1,7 +1,7 @@
 ---
 id: core/connection-from-source-constructor
 name: "Add Connection::from_source constructor (ADR-070 gap — the public extension point)"
-status: pending
+status: completed
 depends_on: []
 scope: narrow
 risk: low