compute: a per-process registry of published index arrangements

Publishing an arrangement produces handles, but a reader on another thread needs
a way to find them. This adds a registry keyed by `GlobalId` and worker ordinal,
shared across all timely workers of a process the way the persist client cache
is, so worker `i` publishes into slot `i` and a reader on worker `i` of another
runtime looks up the same slot. That is sound only because both sides shard keys
by `key.hashed() % peers`, which the equal-peer requirement guarantees.

A slot can be created before it is filled. `get_or_create_placeholder` lets
whichever side touches an id first create the slot and the other adopt it in
place, so a reader never overwrites a publisher's arrangement or imports over a
slot that a later publish replaces. Readers learn about publication and seal
through a dirty-id inbox with a coalescing `SyncActivator` rather than by
polling.

Nothing in the crate calls the registry yet, so it is inert in the same sense as
the shared-trace primitive it builds on. `published_logical_holds` is `pub`
because its callers arrive later and crate scoping would make it unreachable for
dead-code analysis.

Tests are out of line in `sharing/tests.rs`, per the convention in
`src/compute/AGENTS.md`.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: antiguru

src/compute/src/lib.rs

@@ -22,6 +22,7 @@ mod logging;
 mod metrics;
 mod render;
 pub mod shared_trace;
+pub mod sharing;
 /// MV sink machinery, exposed for benchmarks.
 #[cfg(feature = "bench")]
 pub mod sink;