Drive GC payload discovery from ingest journals

[forhappy]

Summary

• Add immutable, batched physical-object intent and completion journals for whole-object ingest windows.
• Add start_gc_journaled for payload candidate discovery without listing the payload namespace.
• Use completion metadata to avoid normal-case per-payload HEAD requests, with intent-based recovery for interrupted uploads.
• Keep legacy start_gc scanning available for direct or pre-journal writers.
• Fix terminal journal pages that omit a continuation token, preventing repeated manifest scans on RustFS/S3-compatible providers.
• Keep payloads whole-object; no packing, chunking, multipart lifecycle, or per-file journal serialization is introduced.

Validation

• Core GC and repository tests
• Full client test suite
• Strict workspace Clippy
• Live RustFS 64-object journaled-GC qualification: completes successfully with zero payload HEAD requests