refactor(core): implement BidiStreamSource trait + AuthContext::anonymous (ADR-070, REQ-CORE-01/02/03)

Extract stream-yield ops from Connection into a BidiStreamSource trait;
Connection now holds Box<dyn BidiStreamSource> instead of the closed
ConnectionKind enum. Three crate-private impls wrap the existing
constructors: QuinnBidiStreamSource (feature quinn), IrohBidiStreamSource
(feature iroh), StreamBidiStreamSource (no gate, yield-once per ADR-065).
Public Connection API is preserved verbatim — handlers dispatch through
the trait object transparently.

The StreamBidiStreamSource::close impl prefixes code/reason with _ and
documents why they're ignored (the drop is the close — ADR-065). This
resolves the ADR-065 leftover clippy warning under --no-default-features
(REQ-CORE-02).

Add AuthContext::anonymous(alpn) convenience constructor (REQ-CORE-03):
sets identity/fingerprint/remote_addr to None, only alpn is populated.
Removes the four-None-field literal that recurred in handler POCs/tests.

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

tasks/core/bidistreamsource-trait.md

@@ -1,7 +1,7 @@
 ---
 id: core/bidistreamsource-trait
 name: "Implement BidiStreamSource trait + AuthContext::anonymous (ADR-070, REQ-CORE-01/02/03)"
-status: pending
+status: completed
 depends_on: []
 scope: broad
 risk: medium