release: v0.2.1 — the hardening (fuzzing adopted + alkcall 0.8.1)

- CHANGELOG: Unreleased cut to 0.2.1 (the fuzz/ workspace: three
  cargo-fuzz targets + stable-toolchain corpus replay in fuzz/shared;
  the two wave-2 diagnostics datagram_backlog/retained_bytes; the
  alkcall 0.8.0 -> 0.8.1 bump — duplicate adopt no longer destroys the
  live channel; compare links updated)
- README: the 0.2.1 hardening note (fuzzing + alkcall 0.8.1 + the two
  additive diagnostics)
- docs/architecture/overview.md: dep tree alkcall 0.8.1
- Cargo.toml: 0.2.0 -> 0.2.1

Verification: cargo test (136 default / 163 with local), fuzz/shared
corpus replay (5 pass), clippy --all-targets -D warnings (default,
all-features, wasm32-unknown-unknown, fuzz/shared), fmt --check, doc,
wasm32 check, and cargo publish --dry-run --allow-dirty all green
(42 packaged files — fuzz/, docs/research|reviews|plans/, tasks/,
.opencode/, AGENTS.md all excluded)

README.md

@@ -300,6 +300,15 @@ enabling it on wasm is a compile error.
   flavor-form op ids in discovery (hub relay wraps them as channels);
   bonus: in-tree `ChannelRelay`/`HubLegTemplate` (alkcall ADR-051).
 
+0.2.1 is the hardening release: the `fuzz/` workspace (three
+cargo-fuzz targets over the datagram codec, the params parsers, and a
+stateful session-pump target; corpora replay as plain `cargo test`) —
+`docs/research/fuzzing.md` — and the alkcall bump 0.8.0 → 0.8.1
+(duplicate adopt/open no longer destroys the live channel — relevant
+to the reverse-path adopt flow). Two additive diagnostics:
+`TunnelSession::datagram_backlog()` and
+`DatagramReader::retained_bytes()`. No API break, no wire change.
+
 ## Features
 
 | Feature | Contents | wasm |