release: prepare 1.0.0-beta.8 - #60

Summary

• replace per-page persistence sorting with a linear ordered collector
• propagate batch metadata inconsistencies as errors instead of panicking
• document ephemeral multi-row batch metadata invariants
• test multi-row variable-length persistence and reload across WorkTablesIndex, Congee, and Arctic
• bump worktable and worktable_codegen to 1.0.0-beta.8

Performance

Release-mode comparison against the prior collector:

• 16 writes: 20% faster
• 256 writes: 25% faster
• 4096 writes: 17% faster (0.192 ms total)

This changes only asynchronous persistence batching. In-memory HFT paths are untouched.

Verification

• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test --workspace
• 173 library tests passed
• 525 integration tests passed, 4 ignored
• 67 codegen tests passed
• 2 doc tests passed