Add duplicate-ID validation diagnostics

[tonyketcham]

Source: flatbread-flow-pmf-audit.md

Project metadata

• Phase: Core relational model and validation
• Sequence: 9
• Priority: P0
• Suggested owner role: backend
• Supporting roles: none
• Dependency status: blocked
• Blocked by: Normalize ID semantics across collections and query args #148 Normalize ID semantics across collections and query args

Task details

• Detect duplicate record IDs within and across the relevant collection loading path.
• Return actionable diagnostics that identify every conflicting source.

Acceptance criteria

• Duplicate IDs fail validation before generated schema/API use.
• Errors include both source locations for the duplicate values.
• Tests cover duplicate IDs in representative flat-file fixtures.