fix(storage): restore multipart disk compression and make the legacy decompressor resumable (#6044)

* fix(storage): restore multipart disk compression and make the legacy decompressor resumable

Multipart uploads have bypassed disk compression since #5169 removed the session marker as a stopgap for mid-stream GET failures. The actual root cause was never the multipart layout: the legacy DecompressReader reset its payload consumption state on every poll re-entry, so a Poll::Pending in the middle of a block payload (routine under the erasure duplex) desynchronized the block framing and surfaced as LZ4 frameType errors. This rewrites the decoder as a resumable state machine, restores the multipart session compression marker, reports logical part sizes in ListParts, and makes the rebalance migration read raw stored bytes so compressed and encrypted objects survive migration verbatim.

Fixes #5957. Internal tracking: backlog#1848, backlog#1850.

* feat(storage): stage multipart compression behind RUSTFS_COMPRESSION_MULTIPART_ENABLED

Review follow-up: a rolling-upgrade window must not create new compressed multipart objects while pre-fix nodes (whose decompressor is not resumable) may still serve reads. The session marker is now additionally gated on RUSTFS_COMPRESSION_MULTIPART_ENABLED, default off, so the restored capability stays dark until the operator confirms fleet convergence. The default flips per the multipart-compression-default-off-window entry in docs/architecture/compat-cleanup-register.md once the minimum supported direct-upgrade release ships the resumable decoder.

* chore(compat): satisfy the cleanup-register guard for the multipart compression switch

The architecture guard requires every backticked identifier in a register entry to carry a RUSTFS_COMPAT_TODO source marker: keep only the entry slug in backticks, and add the marker (with its literal Remove-after condition) at the switch definition.

* chore(rio): drop a dead store in the poison guard and note the end-block branch

Review follow-up: the poison gate re-assigned an already-true flag, and the COMPRESS_TYPE_END branch reads as dead without stating that the writer never emits an end block — that absence is exactly what lets concatenated per-part streams decode as one.

* fix(s3): report empty compressed multipart part size

* fix(s3): report empty encrypted multipart part size

docs/architecture/compat-cleanup-register.md

@@ -33,6 +33,7 @@ for later deletion.
 - `tonic-013-status-render` peer RPC failure classification: internode failures that reach a node only as text (a peer's error_info payload, a status flattened through format!) are classified by matching the rendering of an Unavailable gRPC status. Releases up to 1.0.0-alpha.38 shipped tonic 0.13, which rendered that status as "status: Unavailable, message: ..."; tonic 0.14 renders it as "code: 'The service is currently unavailable', message: ...". Both forms are matched so an older peer's relayed text still marks an unreachable peer offline. Remove the tonic 0.13 form after the minimum supported RustFS peer version ships tonic 0.14 or later.
 - `rustfs-5063` pre-beta.9 Local KMS recovery: persisted Local KMS configs from beta.8 and earlier predate the explicit insecure-development flag, and encrypted key files use the legacy SHA-256 KDF. Remove the config fallback after supported upgrades have rewritten or explicitly resaved all pre-beta.9 configs with the development-default field, and remove the legacy KDF after supported upgrades have rewritten all pre-beta.9 Local KMS key files with explicit at-rest protection.
 - `sse-local-dek-json-v1` legacy local SSE DEK decoding: releases before the JSON envelope wrote wrapped DEKs as `base64(nonce):base64(ciphertext)`, so readers retain that decoder while all new writes use the versioned JSON envelope. Remove the colon decoder after the minimum supported direct-upgrade release writes JSON envelopes and migration tooling has rewritten every retained legacy object.
+- `multipart-compression-default-off-window` staged multipart disk-compression rollout: releases before the resumable legacy decompressor fail transient reads of compressed objects under mid-payload suspension, so multipart uploads advertise the compression marker only when RUSTFS_COMPRESSION_MULTIPART_ENABLED is set in addition to RUSTFS_COMPRESSION_ENABLED, keeping rolling upgrades from creating new compressed multipart objects while pre-fix nodes may still serve reads. Flip the default to enabled (and retire the extra switch) after the minimum supported direct-upgrade release ships the resumable decompressor.
 
 ## Review Checklist