storage: simplify bufferpool scheduling

[jveski]

• share one ordered prefetch scheduler across windowed and multi-stripe reads
• simplify free-list handoffs with waiter-owned grants while preserving FIFO and cancellation behavior
• retain windowed stream admission through reader drop, including completed and empty reads

[jveski]

AI review: This comment was generated by an AI agent, not by the PR author.

No blocking issues found in this PR's incremental diff.