feat: implement getEvents

[RaoulSchaffranek]

Implements the getEvents RPC method per the official stellar-rpc OpenRPC spec.

What changed

• Event capture (src/komet_node/kdist/node.md): the upstream soroban semantics implement the contract_event host function ("x"/"1") as a no-op that drops topics and data. A priority(40) rule now shadows it: it resolves the topics vector and data value from the host objects and stages one JSON record per event in events_staged.jsonl, mirroring how the tracer appends trace records. After a successful sendTransaction the server converts the staged records into one events/events_<ledger>.json per ledger in the spec's Event shape (_finalize_events — base64 SCVal XDR, strkey contract ids, and TOID-style event ids are XDR work K cannot do).
• getEvents dispatch (node.md): validates the requested window against the chain tip (startLedger beyond the tip is -32600 with the same message real stellar-rpc uses), scans the per-ledger event files over the [startLedger, endLedger) window, applies the spec's filters (type, contractIds, topics with * and trailing ** matchers), and paginates with TOID-style cursors. Adds #respondError as the error-shaped counterpart of #respond.
• Parameter validation (server.py): structural checks live next to the existing param validation — filter/topic-segment counts, cursor format, limit 1–10000, xdrFormat (json rejected as unsupported, consistent with the repo's policy), cursor exclusive with startLedger/endLedger.
• scval_from_json (scval.py): inverse of scval_to_json for turning the staged K-side ScVal JSON into stellar_sdk XDR values.
• Docs updated: server.md, architecture.md, node-semantics.md, notes.md (including documented limitations: no system events — the semantics have no source of them; events with values that have no staging representation are dropped with a warning; a topic filter of just ["**"] is rejected; a degenerate window returns an empty result).

Why

getEvents is part of the required stellar-rpc surface and was missing entirely; dapp tooling uses it to ingest contract events. latestLedger and ledger are JSON numbers, topic/value are base64 SCVal XDR, and event ids follow the SEP-35 TOID format, matching real stellar-rpc serialization.

Testing

• New src/tests/integration/test_get_events.py (9 tests) with a dedicated events.wat contract that publishes an event via contract_event: request validation, empty-window shape, the exact Event shape (independently computed XDR, TOID id, strkey contract id, ISO-8601 close time), endLedger exclusivity, filtering by contract id / type / topics, and cursor pagination.
• Full test_server.py suite passes (26 tests, no regressions); make check is clean apart from the two mypy findings already present on main.
• Manually drove the running server over HTTP for edge cases not in the suite: ** topic matchers, cursor+endLedger rejection, limit bounds, degenerate windows, and resuming a persisted io-dir.