feat(io): a segment-keyed IO protocol for planner-driven scans

Replaces the byte-range planning protocol (`vortex_io::request`) with
`vortex_io::segments`, keyed by segment source and source request.

- A request names `IoKey { source: SegmentSourceId, request: SourceRequest }`,
  where `SourceRequest ::= Size | Segment(id)`, with an `IoIntent`: `Fetch`,
  `Prefetch` or `Announce`. One service serves many segment sources, and reads
  resolve to a `SourceResult` (`Size(u64)` or `Bytes(BufferHandle)`).
- `SegmentIoSession::submit` registers a whole batch before any read starts and
  returns one `IoHandle` per request. The session holds fetches and prefetches
  until they finish; an announcement is withdrawn by dropping its handles.
- `poll_ready` reports every fetch's owner once it finishes and drives the
  session's prefetches, through the shared `FetchTracker` helper.
  `SegmentIoService::session` opens one session per scope, and
  `RegisteredSource` is a source's registration with a service.
- vortex-scan: `Planner` and `Morsel` drop the `IoConsumer` supertrait, return
  `NeedsIO(Vec<IoRequest>)`, and receive `deliver(key, result)` once per
  successful fetch.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N4PmceDHg8rFe5bgkhGFU9
Signed-off-by: Claude <noreply@anthropic.com>

Author: claude

vortex-io/src/lib.rs

@@ -23,8 +23,8 @@ mod limit;
 #[cfg(feature = "object_store")]
 pub mod object_store;
 mod read_at;
-pub mod request;
 pub mod runtime;
+pub mod segments;
 pub mod session;
 #[cfg(not(target_arch = "wasm32"))]
 pub mod std_file;