docs: add usage examples and architecture guide (Phase 2)

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Phase 2 — Usage Examples & Architecture Guide

This PR fills in the two stub docs added in the previous PR, replacing the placeholder content with complete documentation.

docs/examples.md — 7 real-world usage scenarios

1. Simple single-table query (ClickHouse) — full working code + generated SQL
2. Time-range filter with time_interval — shows auto-expansion to WHERE clause; includes a second example with extra filters, ORDER BY, and LIMIT
3. Custom metrics formula — explains composition metrics (METRIC_DIVIDE, METRIC_ADD, etc.) with TOML schema + query + generated SQL
4. Multi-table JOIN — fact_dimension_join with two chained dimension tables; schema, query, and generated SQL with JOIN chain
5. RunSync vs RunChan — explains when to use each and the practical trade-offs
6. Debug: BuildSQL — dry-run SQL inspection without executing; also shows Debug: true and SetLogger
7. SQLite quick-start — local-only setup with test/dictionary.sqlite.toml; no ClickHouse needed

docs/architecture.md — contributor guide

• Component responsibilities (Manager, Dictionary/Adapter, Translator, Clause/Statement, Clients, Result)
• Full request lifecycle walkthrough (step-by-step from RunSync to rows)
• Data-flow diagram (ASCII art)
• Package layout table
• Key design decisions (schema-driven, composition by name, GORM as dialect bridge, flat result model)
• Extension points table (new DB, metric type, dimension type, filter operator, adapter, custom fragment)

Part of

Phase 2: Documentation — spec