refactor(server): replace slab ECS with LeftRight-based global metadata

[hubcio]

Centralizing metadata into a single LeftRight store provides lock-free
reads across shards and eliminates coordination complexity inherent in
the distributed slab approach. Shards now read from a shared snapshot
while shard 0 exclusively handles writes, ensuring consistency without
per-read synchronization.

[numinnex]

Overall, this looks good. If you don't have anything else to add there (tests or anything), I think we could merge this.

[hubcio]

@krishvishal @spetz @mmodzelewski mind taking a look?

[krishvishal]

In the add_stream we are currently using 0 as the sentinel value but in add_topic we are using usize:MAX as the sentinel value. This can be inconsistent.

iggy/core/server/src/metadata/writer.rs

Lines 68 to 69 in 0e81d8f

	pub fn add_stream(&mut self, meta: StreamMeta) -> StreamId {
	let assigned_id = Arc::new(AtomicUsize::new(0));

[hubcio]

In the add_stream we are currently using 0 as the sentinel value but in add_topic we are using usize:MAX as the sentinel value. This can be inconsistent.

iggy/core/server/src/metadata/writer.rs

Lines 68 to 69 in 0e81d8f

	pub fn add_stream(&mut self, meta: StreamMeta) -> StreamId {
	let assigned_id = Arc::new(AtomicUsize::new(0));

good catch, fixed