Draft: docs: describe batched item counts with one query per path

What does this MR do and why?

This MR updates ADR 010 and the RPC table for !3034 (merged). One call now counts up to 100 paths. Each path gets its own query. Large paths read on all replicas. A code MR must not change docs. So the docs are here.

Related Issues

• Code: !3034 (merged).
• Design: snippet 6070846.

Testing

The prose lint passes.

Performance Analysis

Docs only.

• This merge request does not introduce any performance regression. If a performance regression is expected, explain why.

Agent context: long-form analysis, file-by-file walkthroughs, profiler output, alternatives considered

• Docs only.

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