feat(cp): copy objects across aliases via download and upload

[loverustfs]

Background

rc cp already performs same-alias S3-to-S3 copies with server-side CopyObject, including recursive prefix copies. Copies between different aliases failed with unsupported_feature, even though rc mirror already downloads and re-uploads across aliases. Operators had to stage through a local path or switch to mirror.

Solution

• Same alias: keep CopyObject / multipart copy.
• Different aliases: HEAD the source, download to a temporary file, upload with the destination alias credentials, then delete the staging file.
• Source content type and user metadata are copied unless --metadata-directive replace is set.
• Storage-class rejection for cross-alias copies uses the upload multipart threshold, because the write is an upload rather than CopyObject.
• Client cache now includes both source and destination aliases.

This is additive. Same-alias CopyObject behavior is unchanged.

This PR must be marked BREAKING because docs/reference/rc/cp.md is a protected CLI behavior contract. The change is additive; no JSON schema or config schema_version bump applies.

Tests

• Unit: planner client aliases for cross-alias and same-alias copies, storage-class threshold, content-type and user-metadata copy vs replace
• Integration: recursive dry-run across aliases without CopyObject; live recursive copy uses LIST + GET + PUT; single-object overwrite uses GET then PUT and never sends x-amz-copy-source; upload forwards x-amz-meta-* from the source HEAD
• Regression: scripts/regression/cross-alias-copy.sh

Validation

cargo fmt --all
cargo clippy --workspace -- -D warnings
cargo test --workspace
./scripts/regression/cross-alias-copy.sh