compute: a shared-trace primitive for cross-thread arrangement reads

An arrangement is normally readable only from the timely worker that maintains
it, because its batches are `Rc`-backed and its trace handle is neither `Send`
nor `Sync`. This adds a publication point that carries `Arc`-backed batches
together with the trace's `since` and `upper`, so a reader on any thread can
mint a `Send` handle for the same arrangement and import it as a snapshot at a
chosen `as_of`. Nothing in the crate calls it yet, so the module is inert: it
compiles, its unit tests exercise publish, import, seal, and compaction
holdback, and no rendered dataflow reaches it.

The concrete `SharedOks*`/`SharedErrs*` type aliases live here rather than
alongside the registry that will consume them. They name a shared-trace handle
over `RowRowSpine` and `ErrSpine` and mention no registry type, so this is where
they belong.

`Published::diagnostics`, `note_writer_logical`, and `note_standing_hold` are
`pub` like the rest of the type's accessors. Scoping them to the crate would
make them unreachable for dead-code analysis while the only callers are the
tests.

Tests are out of line in `shared_trace/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

@@ -21,6 +21,7 @@ mod extensions;
 mod logging;
 mod metrics;
 mod render;
+pub mod shared_trace;
 /// MV sink machinery, exposed for benchmarks.
 #[cfg(feature = "bench")]
 pub mod sink;