fix(validation): align celestial-v1 filename tiling

[zoeyrose]

Summary

Align authored-exit validation with Classic's filename-derived horizontal tiling for celestial_schema 1 maps.

Implementation / behavior

• Parse signed v1 map coordinates using Classic's filename convention.
• Derive only existing horizontal neighbors for empty slots; explicit paths remain authoritative.
• Keep vertical links explicit for reciprocal celestial stack validation.
• Add fixtures for derived, missing, overridden, and both edge-stair landing cases.

Tracks the Classic runtime issue without closing it: atrinik/classic#525

Validation

• Focused authored-exit tests and repository content validation.
• git diff --check.

Limitations / follow-up

This pull request owns static validation; the coordinated Classic pull request owns runtime behavior.

Behavior clarification

Static validation mirrors Classic's general coordinate-filename lookup for every map, regardless of any celestial_schema marker. It fills empty slots only when candidate files exist and leaves explicit paths authoritative. Celestial-specific validation remains separate from this path-discovery rule.