Add four-symbol shape fallback

[platypii]

Summary

• retain the selective 8-symbol ordinary shape layer as the primary regex projection
• add a metadata-gated 4-symbol ordinary fallback for patterns too short to produce an 8-symbol shape
• prefer literal n-grams, then ordinary shape-8, hex shape-8, and finally ordinary shape-4
• extract both ordinary lengths from one shared ring buffer in the existing source traversal
• preserve compatibility with indexes and readers that only understand the singular primary shape metadata

Performance

Measured on the local 25k-row WildChat sample against the shape-8 plus hex index from #10:

• compressed index: 22,065,227 to 22,157,787 bytes (+0.42%)
• posting rows: 12,534,845 to 12,749,559 (+1.71%)
• CPU-only full extraction phase: about +20%; this excludes Parquet read/write time
• MM/DD candidates: 2,500 to 456
• short letter/digit code candidates: 2,500 to 1,401

Long patterns keep the shape-8 results: slash date 76 blocks, SSN 6, phone 42, card 14, fixed IPv4 37, and UUID 49.

Verification

• npm test (145 tests)
• npm run lint