docs: explain adaptive commits and durable ack tiers

[nwoolmer]

Summary

Documents adaptive commit mode and the tiered QWP durable-ack protocol introduced by questdb/questdb#7411 and questdb/java-questdb-client#98.

Commit modes

• compares nosync, async, sync, and adaptive
• explains the adaptive WAL / lazy apply / durable epoch model
• documents the group-commit RPO, including background sweep scheduling delay
• covers all adaptive tuning keys and when to use each mode
• distinguishes WAL and non-WAL guarantees
• documents migration and rollback safety:
  • switching commit modes on an adaptive-capable version is supported across restarts
  • rolling back to a pre-adaptive binary is a separate, currently unverified cross-version path and must not be attempted directly after an unclean adaptive shutdown

QWP durable acknowledgements

• adds local, replicated, local,replicated, and legacy true negotiation
• documents exact/all-or-nothing grant validation
• specifies STATUS_LOCAL_DURABLE_ACK (0x0E) and the shared payload layout
• defines strongest-tier store-and-forward trimming
• explains that local progress requires adaptive WAL tables
• documents the passive-server keepalive requirement and final watermark drain before close
• marks the combined tier as protocol-defined but not currently granted by servers

Related connect-string, Java, C/C++, Rust, Python, delivery, and store-and-forward pages are updated for consistency.

Test plan

• yarn build
  • Docusaurus production build passed, including broken-link/anchor validation
• git diff --check

[github-actions]

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