feat(python): rollout store bindings (local + remote)

[beinan]

Summary

The rollout store shipped in 0.6.0 with Rust core / client / server support but no Python bindings — Python users couldn't append or read rollouts at all, local or remote. This adds both, mirroring the existing Context / RemoteContext pattern.

• RolloutStore (sync): embedded via open(uri), or remote via connect(base_url, name) / connect_or_create(...).
• RemoteRolloutStore (async): awaitable HTTP wrapper (blocking calls run in an executor), the path RL generation workers + the learner use.

Both dispatch through the unified lance-context RolloutStore enum, so a single pyclass covers embedded and remote.

Ergonomics

Rollout rows carry ~35 fields, so records cross the FFI boundary as JSON (serde on the Rust side) rather than 35 keyword args. The Python wrapper accepts:

store.add({"id": "r0", "rollout_id": "traj-1", "reward": 1.0})   # one dict
store.add([{...}, {...}])                                          # many
store.add_one(id="r0", rollout_id="traj-1", reward=1.0)           # kwargs

binary_payload is accepted as raw bytes and base64-encoded to match the DTO's JSON contract; get/list project it out (cheap scans) and get_blob(id) materializes it on demand.

Methods: open / connect / connect_or_create, add, add_one, list, get, get_blob, version, checkout. Exported from lance_context.

Changes

• python/src/lib.rs: RolloutStore pyclass wrapping the unified enum; JSON marshalling of records/DTOs; registered in the module.
• python/python/lance_context/api.py: RolloutStore (sync) + RemoteRolloutStore (async) wrapper classes.
• python/python/lance_context/__init__.py: exports.
• crates/lance-context/src/lib.rs: re-export CreateRolloutStoreRequest (needed for connect_or_create).
• python/Cargo.toml: depend on lance-context with the remote feature.

Test plan

• test_rollout.py — embedded: dict/list/kwargs add, list/get, blob roundtrip (projected out, fetched on demand), reopen sees prior rows, empty-add rejected
• test_rollout_remote.py — spawns a real lance-context-server subprocess, async connect_or_create + full roundtrip + cross-connection read-your-writes; skips cleanly if the server binary isn't built
• ruff check + ruff format --check + pyright clean; 7/7 tests pass

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