docs(bf-2i8zv): add YAML footguns and profiles validation guide
Add docs/research/profile-authoring.md documenting YAML authoring
gotchas for document profiles and the pdftract profiles validate
command.
Coverage:
- YAML footguns: scalar type coercion (on, yes, true), significant
indentation, multi-line strings
- The match: combinator DSL (all:, any:, none:) with correct
and incorrect examples
- The fields: DSL (near:, region:, pick:, regex:, parse:)
- Common validation errors and pdftract profiles validate messages
- Profile search path resolution order
- PROFILE_SECRETS_FORBIDDEN constraint
Acceptance criteria:
- File exists with all 6 topic areas covered ✓
- At least 3 annotated YAML examples ✓
Required by ADR-007 (plan line 480).
ef3f4929e3