Add missing-reference validation diagnostics

[tonyketcham]

Source: flatbread-flow-pmf-audit.md

Project metadata

• Phase: Core relational model and validation
• Sequence: 8
• Priority: P0
• Suggested owner role: backend
• Supporting roles: none
• Dependency status: blocked
• Blocked by: Define Flatbread relational primitives glossary #143 Define Flatbread relational primitives glossary; Normalize ID semantics across collections and query args #148 Normalize ID semantics across collections and query args

Task details

• Fail fast when flat-file data references records that do not exist.
• Make the diagnostic useful enough for developers to fix the source file without tracing internals.

Acceptance criteria

• Missing relation targets fail validation before query-time surprises.
• Errors include collection, file or source location, field, and missing target context.
• Tests cover missing references for the posts/authors/tags style relation model.