docs(rfc): define sandbox workload identity for shared supervisors

[dhirajsb]

Summary

RFC 0018 requires per-sandbox credential isolation but does not specify how a shared Supervisor receives the Sandbox Pod's workload identity. Add a workload identity section requiring the Sandbox-side gRPC client to obtain its pod-specific SVID through the local SPIFFE Workload API UDS and forward it to the Supervisor for dynamic Credential Provider exchange.

This follow-up targets the RFC branch in this fork so it can be merged into NVIDIA#4377. The diff contains only the workload identity addition and section renumbering.

Related Issue

Refs NVIDIA#4311 and NVIDIA#4377. This documentation contribution does not complete the supervisor implementation tracked by NVIDIA#4311.

Changes

• Add the UDS → Sandbox client → shared Supervisor → Credential Provider sequence diagram.
• Require validation and binding to the sandbox/run session and ownership epoch, preserving the Sandbox identity during credential exchange.
• Define rotation, handover, credential-cache isolation, and failure behavior; keep X.509 private keys in the Sandbox Pod.

Testing

• Checks appropriate to the affected code and behavior pass
• Markdown lint (markdownlint-cli2 0.22.0): 149 files, zero errors.
• Repository Mermaid parser: all diagrams in the RFC valid.
• Mermaid CLI: all three RFC diagrams rendered successfully.
• git diff --check: passed.
• Unit and E2E tests are not applicable to this RFC-only change.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (RFC 0018)

Commit is also SSH-signed.