docs: simplify architecture diagram to layers only

[Copilot]

The architecture diagram was overloaded — 13 nodes, 20+ mixed-type arrows (uses + inheritance), and a utilities subgraph — making it hard to extract signal about how the layers relate.

Changes

• Collapsed variants: Each layer (Storage, Partition, Index) is now a single node. The Writable/Readable/ReadOnly split is already covered by the text and tables immediately surrounding the diagram.
• Removed utilities subgraph: Clock, Watcher, WatchesFile, IndexEntry removed from diagram; they remain documented in the component descriptions.
• Removed JoinEventStream: Omitted to keep focus on the core stream concepts.
• Labelled arrows: Every arrow carries a caption describing the relationship (e.g. "writes to stream=partition", "exposes", "made up of multiple", "iterates", "persists state").
• Write vs. read path distinction: EventStore →|"writes to stream=partition"| Storage surfaces the write-stream concept (a stream name maps to a specific partition). EventStream only connects to Index ("iterates"), reflecting that a read stream traverses an index independently of which partitions it visits.
• Storage node annotated: "single ordered log" appears as an italic subline inside the Storage node, conveying how EventStore accesses it.
• Consumer exposed by EventStore: Consumer is shown as exposed by EventStore with a "persists state" arrow to a Disk node.
• Index arrows reversed and annotated: Index →|"primary index (over all partitions)"| Storage and Index →|"secondary index (stream) over 1-n"| Partition, making it clear that a single index can cover 1-n partitions.

---

⚡ Quickly spin up Copilot coding agent tasks from anywhere on your macOS or Windows machine with Raycast.

[albe]

@copilot work on my last review comments

[albe]

@copilot the arrow between consumer and EventStream crossing the Eventstore to EventStream arrow is suboptimal. Try to find a solution, but if mermaid doesn't allow to fix that, instead remove the consumer->EventStream arrow again.

More importantly, I'd like to show the subtle distinction between write stream, which is just a name that translates to a specific partition in the commit/write path of EventStore, vs the readonly EventStream which is the class exposed by EventStore. Therefore it makes sense to remove the EventStream->Partition link, as the read stream iterates an index, independent of the partitions it might visit. Meanwhile an EventStore.commit will write to one partition only in the default partitioning scheme.

[Copilot]

@copilot the arrow between consumer and EventStream crossing the Eventstore to EventStream arrow is suboptimal. Try to find a solution, but if mermaid doesn't allow to fix that, inst...

Done in 45edb0f:

• Removed Consumer → EventStream (no more crossing arrow)
• Removed EventStream → Partition (read stream iterates index, independent of which partitions it visits)
• Added EventStore →|"writes to (default partition)"| Partition to make the write path explicit

[coveralls]

coverage: 98.004%. remained the same
when pulling 843c508 on copilot/simplify-architecture-diagram
into 04fbba6 on main.