backup: S3 encoder for buckets, objects, and blob reassembly (Phase 0a)

[bootjp]

Summary

Stacked on top of #716 (DynamoDB) and #717 (s3keys.ParseBlobKey). Adds the S3 encoder for the Phase 0 logical-backup decoder — the most complex per-adapter piece because it must reassemble multipart object bodies from independent blob chunks.

Snapshot prefixes handled:

• !s3|bucket|meta|<bucket> → s3/<bucket>/_bucket.json
• !s3|obj|head|<bucket><gen><object> → s3/<bucket>/<object>.elastickv-meta.json (sidecar)
• !s3|blob|<bucket><gen><object><uploadID><partNo><chunkNo>[<partVersion>] → spilled to scratch on arrival; concatenated into s3/<bucket>/<object> at Finalize in (partNo, partVersion, chunkNo) order
• !s3|upload|meta|/!s3|upload|part|: excluded by default; opt in via --include-incomplete-uploads to emit under _incomplete_uploads/records.jsonl
• !s3|bucket|gen|, !s3|gc|upload|, !s3route|: ignored

How body assembly works

Each blob chunk is written atomically to a per-(bucket, object) scratch directory as it arrives, registered under the (uploadID, partNo, chunkNo, partVersion) routing key. At Finalize, chunks for each object are sorted and concatenated into a single body file via tmp+rename. Body bytes are never held in memory — only the scratch path map. The scratch tree is removed on Finalize.

Reserved-suffix collision handling

A user object key ending in .elastickv-meta.json is rejected with ErrS3MetaSuffixCollision by default. WithRenameCollisions(true) appends .user-data to the body file and records the rename in s3/<bucket>/KEYMAP.jsonl with KindMetaCollision so the dump remains reversible. Per-bucket KEYMAP.jsonl is opened lazily and dropped on Finalize if no records were written.

Test plan

• go test -race ./internal/backup/... — pass.
• golangci-lint run ./internal/backup/... — clean.
• 11 tests covering bucket-meta round-trip, multipart out-of-order chunk reassembly, orphan-chunks warning, sidecar collision (rejected/renamed), JSON-decode rejection on both paths, ignored-prefix no-op, --include-incomplete-uploads round-trip + default skip, versioned blob assembly.

Stacking

Base: feat/backup-phase0a-dynamodb (PR #716). Also depends on feat/s3keys-parse-blob-key (PR #717).