fix: support sparse PCR banks

[leopoldjoy]

Summary

• validate PCR indices against the fixed 0..31 bank rather than the number of entries
• return a 32-slot PCR bank with explicit CBOR-null sentinels for omitted indices
• add sparse-map, duplicate, out-of-range, and consumer documentation coverage

Audit finding

Addresses external finding #1.

Tests

• forge fmt --check
• forge test (208 passed, 1 skipped)
• forge build --sizes
• NitroValidator runtime: 17,460 bytes