docs: rename slippage accounting to buffer

[jean-neiverth]

Summary

Aligns docs with the naming changes from the feat/slippage-balance-endpoint branch in byos-service-ts (commits 096d508, e86c0eb, 444e297):

• slippage → buffer in all accounting contexts: table names (slippage_entries → buffer_entries), endpoint (GET /slippage-balance → GET /buffer-balance), audit events (slippageDebited → bufferDebited), and terminology ("loose slippage" → "loose buffer", "slippage clearing" → "buffer clearing")
• eth_amount → native_token_amount: all references to "converted to ETH" updated to "converted to native token"
• buy_token field: added to the buffer_entries table description in service.md
• Over-delivery credits are never paid out: removed all references to collateral deposits for over-delivery credits — credits now only offset future shortfalls

References to CoW protocol-level slippage (settlement slippage returned weekly, buffer usage) are intentionally unchanged — those refer to a different concept.

Files changed

File	Changes
design-document.md	Renamed #post-settlement-slippage-accounting anchor to #post-settlement-buffer-accounting; updated terminology and credit semantics throughout
service.md	Updated listener table, penalty loop description, and persistence table
contracts.md	Updated escrow event description and "How BYOS interacts" section
guides/sub-solver-integration.md	Updated risk section, API endpoints table, and pre-launch checklist
glossary.md	Updated "Floor and ceiling" entry

Test plan

• Verify all internal anchor links resolve correctly (especially #post-settlement-buffer-accounting)
• Confirm no remaining slippage references in BYOS-accounting context (CoW-level slippage references are intentional)
• Cross-check terminology against byos-service-ts feat/slippage-balance-endpoint branch

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