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>

tines-sim/Cargo.toml

@@ -1,13 +1,13 @@
 [package]
 name = "tines-sim"
-version = "0.1.0"
+version = "0.2.0"
 edition = "2021"
 license = "Apache-2.0"
 description = "Discrete-time simulator for tines: contact traces, convergence scenarios, and property tests."
 authors = ["K. Isom"]
 
 [dependencies]
-tines-core = { path = "../tines-core", version = "0.1.0" }
+tines-core = { path = "../tines-core", version = "0.2.0" }
 rand = "0.8"
 
 [dev-dependencies]