fix: scroll sequential walks were racing and re-reading page one

[tellet-q]

• Every walk started at the top. A fresh walk was opened with offset: None, which means "from the beginning of the collection." So all concurrent walks began at point 0 and marched through the same pages in lockstep — the first fetched each page from disk, the rest got cache hits. Walks now open at a random point, so concurrent walks cover different stretches and each page is genuinely read once.
• Drops the --rps guard. It was added to block mode: sequential with --rps, purely because unbounded concurrency guaranteed the slot collision. The pool removes the race, so the restriction — and its now-false comment — is gone; --rps runs sequential correctly, with the walk count simply floating with the in-flight request count.

before:
walk 0 ─┐
walk 1 ─┼─ all from point 0 ──▶ 1 disk read, N−1 cache hits
walk N ─┘

after:
walk 0 ──── from random point A ──▶
walk 1 ──── from random point B ──▶ each page read once, from disk
walk N ──── from random point C ──▶

[Copilot]

Pull request overview

This PR fixes sequential scroll concurrency so concurrent walks don’t all start from offset None (page one), which previously caused lockstep re-reads and cache hits instead of genuine disk reads. It also removes the previous --rps + mode: sequential prohibition by introducing a walk pool that prevents sharing a cursor between concurrent requests.

Changes:

• Replaces per-worker fixed walk slots with a mutex-protected pool of in-progress walks that are checked out per request and returned advanced.
• Seeds new sequential walks from a random point (via a sample: random query) to avoid concurrent walks re-reading the same initial pages.
• Removes the --rps guard for sequential mode and updates docs/examples to reflect the new sequential behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/scroll.rs	Implements the walk pool and random seeding for sequential mode; removes slot-based walk sharing.
src/main.rs	Drops the --rps + sequential-mode bail-out now that walks are no longer shared by slot.
README.md	Updates sequential mode documentation to describe random seeding / reduced overlap for concurrent walks.
examples/scroll-config.yaml	Updates config comments to match the new sequential walk behavior.

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