validation: enforce filter semantic checks from spec rule 18/33

[pesap]

Problem

arco validate passes filters that produce empty subsets due unresolved RHS identifiers.
No warning/error emitted.

Spec expectations:

• Rule 18: detect unknown filter references.
• Rule 33: warn when filtered subset is empty.

Repro

filter { tech == wind } with no wind column, only value in tech.
arco validate currently reports success.

Acceptance criteria

• Validation flags unresolved/invalid filter symbol usage with actionable message.
• Validation warns on empty filtered subsets (name + parent set + predicate).
• Diagnostic includes source location and fix hint:
  • suggest quotes for categorical literals where applicable.