GS: ClickHouse 26.8+ commit-cursor append refresh

Parent: #398
Depends on: #400, #401, #402

Design:

• docs/design/google-sheets/append-refresh.md
• docs/design/google-sheets/decisions.md

Scope

Implement capability-gated append-only refresh for eligible direct MergeTree/ReplicatedMergeTree sources using per-partition persistent _block_number and _block_offset checkpoints and finite bounded SELECT queries.

Acceptance criteria

• Append UI requires configured minimum version (initially 26.8) and successful runtime capability probes.
• Eligible query validation permits one deterministic direct source and rejects joins, views, FINAL, aggregation, DISTINCT, window functions, UNION, table functions, and business LIMIT.
• Binding pins table UUID, query hash, schema hash, source-subset parameters, and initialization mode.
• Initial modes support history export and future-only checkpoints.
• Refresh captures finite per-partition upper boundaries.
• Pages are ordered and filtered by _block_number / _block_offset and use decimal-string UInt64 values.
• Each Google commit copies exact staged rows and advances only the corresponding partition checkpoint in the same publication batch.
• A closed tab resumes from the last committed page without duplicates.
• Invalid capability, UUID, query, schema, parameter, row-count, or checkpoint state requires explicit full refresh.
• SELECT ... STREAM, updates, deletes, and silent fallback are absent.

Concurrency

Use advisory lease metadata and generation rechecks. Strict distributed locking is out of scope.

Comienza leyendo docs/design/google-sheets/append-refresh.md y docs/design/google-sheets/decisions.md; después revisa las dependencias #400, #401 y #402. Usa los criterios de aceptación indicados para definir cuándo se ha completado, incluida la validación condicionada por capacidades, los checkpoints de partición persistentes, las páginas de refresh acotadas, los commits exactos de Google, el comportamiento de reanudación y el manejo explícito de full-refresh para un estado no válido.