BRD Generator: improve system prompt, diagram handling, and depth directives
- Enhanced system prompt with natural language guidelines (rule 10)
- Diagram schema now requires source_text quotes from CLIENT INPUT for every node/edge
- Depth directives changed from 'return N or more' to 'aim for up to N' to prevent padding
- Added detailed diagram validation: decision diamonds need labelled branches, end nodes cannot have outgoing edges, disconnected steps rejected
- coerceSection: split pointers on newlines only (not sentences) to preserve abbreviations/decimals
- sectionIssues: now validates diagrams with diagramIssues() using sourceText for source_text verification
- generateSection: passes authoritative input+facts to validation for cross-checking
- retriever.js: minor fix
- server.js: related updates
- Updated tests for generator, generation-quality, and diagram