release: v0.3.0 — the dependency-major (alkcall 0.9.0)

- Cargo.toml: alkcall 0.8.1 -> 0.9.0 (crates.io, the stub-sweep
  release) and 0.2.1 -> 0.3.0 (breaking dep bump + public alkcall
  re-exports -> minor, matching the 0.7.1 -> 0.8.0 ride on 0.2.0)
- fuzz/shared/Cargo.toml: alkcall 0.9.0 (path-linked alktunnels must
  see one alkcall version)
- src/producer.rs + tests/producer_direct_op.rs: the only code contact
  with the 0.9.0 breaking surface — Establishment no longer derives
  Debug; two expect_err calls -> .err().expect, one untyped
  panic!("{other:?}") fall-through -> typed Err/Ok arms
- CHANGELOG: 0.3.0 section + compare links
- README: the 0.3.0 note
- docs/architecture/overview.md: dep tree alkcall 0.9.0

Verification: cargo test (136 default / 163 with local), fuzz/shared
corpus replay (0 pass, empty target set — no regressions), clippy
--all-targets -D warnings (native + wasm32-unknown-unknown), fmt
--check, wasm32 check, and cargo publish --dry-run --allow-dirty all
green

Cargo.lock

@@ -27,9 +27,9 @@ dependencies = [
 
 [[package]]
 name = "alkcall"
-version = "0.8.1"
+version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "bf6af1e4af1460480849adf959c972d65deeafc36f4101064ced1ce6d1adfd5b"
+checksum = "a3562f1f7b348cdeb3d9181eaf57b0a2fb3f988d4280e8e225dadfd7f763cb84"
 dependencies = [
  "async-trait",
  "bytes",
@@ -48,7 +48,7 @@ dependencies = [
 
 [[package]]
 name = "alktunnels"
-version = "0.2.1"
+version = "0.3.0"
 dependencies = [
  "alkcall",
  "bytes",