chore: release 0.2.0; document the store contract and on_send_error

Bump tines-core and tines-sim to 0.2.0 (Peer::in_flight changed type).
Add CHANGELOG.md. README gains a "Writing a store" section and a note on
calling Exchange::on_send_error when driving Exchange without Node.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.lock

@@ -548,7 +548,7 @@ dependencies = [
 
 [[package]]
 name = "tines-core"
-version = "0.1.0"
+version = "0.2.0"
 dependencies = [
  "blake2",
  "heapless",
@@ -558,7 +558,7 @@ dependencies = [
 
 [[package]]
 name = "tines-sim"
-version = "0.1.0"
+version = "0.2.0"
 dependencies = [
  "proptest",
  "rand 0.8.8",