Compatibility Lens

• Internal metadata uses metadata_compat helpers for dual RustFS/MinIO keys, including mixed casing and removal of both twins.
• Binary UUID metadata treats absent, empty, and nil as no value. Unversioned remote tiers receive no versionId; versioned purge requests retain the real version ID.
• xl.meta changes preserve supported header/meta versions, recompute signatures, decode legacy fixtures, and remain readable by old RustFS/MinIO.
• Foreign/corrupt metadata validates parallel array lengths and missing fields; it returns a decode error rather than indexing, panicking, or fabricating data.
• Do not “correct” byte-for-byte MinIO ports without legacy fixture evidence. Bitrot framing, shard math, distribution, and inline prefixes are contracts.
• Client-visible metadata/events strip both internal prefixes case-insensitively.
• Proto fields are appended, never reused/renumbered; FlatBuffers tables extend compatibly and absent new fields fail closed where authorization/quorum is involved.
• Replay real client request shapes and exact pagination boundaries for S3 handler changes.
• Bucket metadata/IAM/config parsing remains compatible with pinned real MinIO fixtures and encrypted migration data.
• Compatibility shims use RUSTFS_COMPAT_TODO(<task-id>), have a removal condition, and default toward reading old data safely.