fix(store): uniform compose part sizes for Cloudflare R2

[gonewx]

Problem

Bundle builds against Cloudflare R2 fail forever with:

bundle task: store error: s3 complete multipart: service error

(repeats every maintenance pass; the server log shows receive-pack-side pushes are fine, only compose fails at CompleteMultipartUpload).

Root cause

R2 requires every non-trailing part of a multipart upload to have the same length (it answers InvalidPart: All non-trailing parts must have the same length.), whereas S3 only requires non-trailing parts to be >= 5 MiB.

compose cut copy ranges at COPY_PART = 1 GiB. For the standard bundle shape — a small bundle header in front of a large pack — the layout is one 5 MiB uploaded part (header + pack prefix) followed by one large copied part (the rest of the pack), so R2 rejects the Complete. The contract test's compose case only exercises a ~1 MiB trailing copy, a layout that happens to be legal, which is why this never showed up before.

Fix

Cut copy parts at MIN_PART instead of 1 GiB, so every non-trailing part — uploaded or copied — is exactly 5 MiB and only the trailing part may differ. S3 accepts uniform parts just as well; part counts stay small (a 5 GiB compose needs <= 1024 copy requests, a 30 GB one ~6k, below the 10k part limit).

Verification

• Reproduced against a real R2 bucket with a raw-SDK probe (31-byte header + 46 MiB source → UploadPart(5 MiB) + UploadPartCopy(41 MB)): before the fix Complete fails with InvalidPart, after it succeeds and the content round-trips (head, tail, total size).
• Full store contract suite against R2 (virtual-hosted style, region auto): all cases pass, including test_compose.
• Production: a 48.7 MiB weekly bundle now builds successfully on R2 (bundles/list.pb published).

Second commit is a small test affordance: s3_contract hardcodes path-style addressing and us-east-1, which makes it impossible to run the suite against R2/OSS; the two env overrides keep rustfs defaults unchanged.

[0bserver07]

The R2 constraint is real (every non-trailing part the same size), but making COPY_PART 5 MiB everywhere changes the cost of the thing compose exists for: the weekly full bundle of a 32 GB base goes from ~32 UploadPartCopy requests at 1 GiB parts to ~6,500, and the docs/ROUNDTRIPS.md budget table still lists compose at about one request per source. S3 and GCS don't need uniform parts, so this pays a ~200x request increase on stores that didn't ask for it. Maybe a store.s3.compose_part_size (default 1 GiB, 5 MiB for R2) plus a ROUNDTRIPS row? The env-var additions to the contract test look handy either way.