[Feat]Wa dump blockwise

[wuhuxiao]

Purpose

Add configurable block-wise WA KV cache dumping for the FAWA connector.

Previously, WA cache dumping only persisted the final WA tail block for each chunk prefill. This made WA external-cache reuse depend on chunk boundaries and could miss intermediate canonical block boundaries. This PR adds an option to dump WA cache block-wise so each canonical hash block can persist its corresponding WA tail state.

Modifications

• Add wa_dump_block_wise config option, defaulting to true.
• Update FAWA dispatch metadata generation to support two WA dump modes:
  • block-wise WA dump: dump WA tail rows for every canonical block boundary.
  • chunk-wise WA dump: keep the old behavior and dump only the final WA tail per chunk.
• Keep WA load behavior loading only the final matched WA boundary.
• Update TP dump slicing so block-wise WA dump follows the same TP key partitioning as FA dump.
• Split configured posix_capacity_gb between FA and WA stores.
• Document wa_dump_block_wise in examples/ucm_config_example.yaml.

Test

[ygwpz]

💡 Suggestion: This division of posix_capacity_gb by 2 needs documentation. Why is the capacity halved? Is it to split capacity between FA and WA stores? Consider adding a comment explaining the rationale to avoid confusion.