release v0.2.1

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Release 0.2.1

Generated by the Release prep workflow. Merging this PR creates annotated tag v0.2.1 and runs the Release workflow: verify -> GitHub Release -> crates.io publish (Trusted Publishing OIDC, tokenless).

Changelog

[0.2.1] - 2026-09-14

Changed

• The runtime platform matrix is documented as a pointer to the upstream published wheel list rather than a fixed enumeration, so the targets a consumer is told about stay in step with what upstream actually publishes (Windows x64 included; macOS needs the sibling -spawn-helper).

Fixed

• RunResult::final_response now falls back to an earlier root assistant/message
  when the last one is malformed (non-object data or non-array content),
  matching the Python SDK's final_response reversed-scan algorithm. The
  previous implementation returned "" instead of trying an earlier event.
• Corrected the public rustdoc on RunResult::final_response and the normative
  spec §6.2, which both incorrectly claimed final_response "never falls back
  to an earlier event" while citing the Python source that does fall back.
  The behavior is unreachable from a conformant runtime (which always emits
  message.content as a JSON array) but the algorithm and docs now agree.
• RunResult::final_response treats a non-string text block (including
  null) as "". That is a recorded divergence from the Python SDK, which
  coerces a truthy non-string via str() (42 → "42", true → "True").
  There is no runtime behavior change.
• A HarnessClient dropped without an explicit close() now closes its notification channel, so a subscription awaiting the next notification returns Error::TransportClosed promptly instead of only when the aborted background reader task happens to be torn down. That error reports the closed reason, plus the runtime's exit code and captured stderr tail when they were observed.
• Config::dsh_bin is the current runtime-binary override name (renamed from Config::runtime_bin; the DSH_RUNTIME_BIN environment route is preserved).
• Runtime acquisition now documents all five targets upstream publishes for the
  runtime — Linux x64, Linux arm64, macOS arm64, macOS x64, and Windows
  x64 — and no longer states that macOS x64 has no wheel. A macOS x64 wheel
  is published, so Route B (the self-contained wheel) works there; Route C
  (build from source) remains the route that reproduces the exact contract
  basis this crate was verified against, and the fallback for any platform with
  no published wheel.
• Session::run no longer wedges indefinitely when the runtime dies mid-turn (stdout EOF after session/prompt succeeds but before the inbox receipt or root-idle notification arrives). The high-level API surfaces Error::TransportClosed with the closed reason and the stderr tail the end-of-stream path captured, plus the exit code when the best-effort poll of the exiting child observed it, as its doc contract always promised ("once the channel (or the client) is closed, Error::TransportClosed is returned").
• NotificationSubscription::recv parked in broadcast::Receiver::recv().await now wakes with TransportClosed when the runtime dies spontaneously (stdout EOF without an explicit close), instead of hanging forever. The fix shares the original broadcast Sender between HarnessClient and the read loop and drops it on the read loop's EOF path, so the channel closes and a parked recv resolves with RecvError::Closed. Subscriptions created after runtime death remain born-failed.