feat(rc-ops)!: add health, readiness, and usage commands

[cxymds]

Closes rustfs/backlog#1377

Background and user impact

The CLI did not expose RustFS liveness/readiness probes or the server's background-scanner data-usage snapshot. Operators had to combine curl, admin credentials, and custom S3 listing scripts, and a client-side usage scan could become unexpectedly expensive if started implicitly.

What changed

• Adds rc ping for bounded unsigned GET /health liveness checks.
• Adds rc ready for bounded unsigned GET /health/ready dependency-readiness checks.
• Adds rc du ALIAS[/BUCKET[/PREFIX]].
• Uses the signed RustFS GET /rustfs/admin/v3/datausageinfo snapshot when supported.
• Requires explicit --fallback before any slower client-side S3 scan.
• Supports paginated current objects, versions/delete markers, incomplete multipart uploads, and uploaded parts.
• Reports sanitized human output and schema-v3 JSON with partial-result semantics.

Review resolution

• Guards current-object pagination against missing, empty, or non-advancing continuation tokens, returning a bounded network error instead of looping.
• Adds regression coverage for each invalid-token case.
• Adds an end-to-end du test asserting unsupported-feature exit code 7 and verifying that fallback scanning is not started.
• Keeps readiness failure and transport failure intentionally retryable as exit code 3; their messages retain the operational distinction.
• Does not independently change the workspace package version: the repository's release/tag stream is already ahead of 0.1.24, while this PR preserves output v1/v2 and adds only the explicitly versioned v3 families. The protected-file gate is satisfied through the BREAKING marker and migration guidance below.

RustFS compatibility and safety

• /health and /health/ready are public operational endpoints; credentials are never attached.
• The data-usage route is signed and capability-gated.
• Prefix and incomplete-upload dimensions absent from the snapshot require explicit S3 fallback.
• Health timeouts are bounded to 1–300 seconds.
• Authentication failures remain exit code 4 and never trigger a scan unless --fallback was explicit.
• Alias-wide scan failures preserve successful buckets, mark output partial, and exit 3.
• Totals use saturating arithmetic and server-controlled text is sanitized before human output.

BREAKING contract note

This PR adds command contracts under the protected docs/reference/rc/ path and is therefore marked BREAKING per repository policy. Existing commands, exit codes, config schema, and output v1/v2 remain unchanged. Consumers of the new commands should select the output-v3 health or usage family.

Validation

• cargo fmt --all --check: passed.
• cargo clippy --workspace -- -D warnings: passed with zero warnings.
• cargo test --workspace: passed.
• Compatibility script unit tests and repository consistency check: passed.
• Focused object pagination, du exit-code, output-schema, replication, and launcher tests: passed after integration with latest main.

[cxymds]

This PR supersedes #275 after its base branch was deleted. It carries the prior review fixes: bounded current-object pagination for missing, empty, and repeated tokens; regression tests for those cases; and an end-to-end unsupported-feature exit-code 7 test that verifies no fallback scan occurs. The branch has been integrated with latest main and the full mandatory validation suite passes. Please re-review the migrated changes.