Commit 1f41403 showed base64 padding rules were accidentally relaxed by base64url work; 7575215 showed uuidv7(interval) accepted infinities/out-of-range shifts. Add encode/decode round-trip identity tests for lengths 0..N, invalid-padding rejection for both variants, and uuidv7 interval boundary tests (±infinity, extreme values, ordering property). Only add what existing tests don't already cover.
Commit 1f41403 showed base64 padding rules were accidentally relaxed by base64url work; 7575215 showed uuidv7(interval) accepted infinities/out-of-range shifts. Add encode/decode round-trip identity tests for lengths 0..N, invalid-padding rejection for both variants, and uuidv7 interval boundary tests (±infinity, extreme values, ordering property). Only add what existing tests don't already cover.