feat: add optional streaming object storage capability

[cmilesio]

What

Add an optional storagecore.ObjectStorage capability and storage.Objects(disk) discovery helper. The S3 driver supports paged object iteration, streaming reads and writes, content metadata, version-pinned reads, and ETag conditions. Opaque object keys remain relative to the disk prefix and retain dot segments and repeated slashes.

Keep existing Storage, buffered Get/Put, path semantics, other drivers, and minimum Go versions unchanged. Drivers without streaming support return ErrUnsupported instead of buffering an entire object as a fallback. Stream callers close readers and provide an explicit size for writes. Bucket provisioning is opt-in through s3storage.PrepareBucketContext, using existing driver configuration and refusing to create after a permission failure.

Why

Framework application backups need bounded object copies that preserve metadata and detect source changes. These provider operations belong in the storage library rather than a second S3 implementation in the backup package. GoForj PR #246 consumes this capability while retaining archive format and checksum ownership.

Unit and race tests cover streamed round trips, opaque keys, metadata, conditional reads, cancellation, callback and provider failures, invalid pagination and prefix escapes, and missing versus forbidden buckets. Root and S3 module tests also passed with GOWORK=off against the published core commit.

An independent reviewer agent examined the change and reproduced three issues: XML-sensitive object keys, non-seekable HTTP uploads, and cancellation during SDK prehashing. Follow-up fixes request URL-encoded listings and decode keys exactly once, preserve explicit payload-signing policy while supporting opted-in HTTP streams, and stop further source reads on cancellation. Endpoint handling covers normalized schemes and AWS endpoint overrides. The independent re-review reported no remaining actionable findings.

Protocol tests exercise actual SDK requests against local HTTP and HTTPS servers, including retained HTTPS streaming checksums. The S3 race suite and vet pass, and root/core tests pass with Go 1.24.4 and GOWORK=off. Object operations and bucket preparation have 100% local statement coverage. This review did not exercise a live AWS account.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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