Add stateless backend-scoped task handles

[lucarlig]

Summary

• add a versioned stateless task-handle codec for MCP 2026-07-28
• encrypt caller, virtual-host, backend, and upstream task context with AES-256-GCM
• validate JWT subject scope and current backend membership with indistinguishable invalid-handle errors
• configure a shared replica key and document the contract tersely in _context/wiki

The control plane remains the authentication authority; the dataplane only validates its JWT and binds the opaque handle to the validated sub. This PR is the codec prerequisite. Task method proxying remains in the follow-up issues.

Validation

• cargo fmt --all -- --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo nextest run --locked --workspace — 186 passed
• cargo deny check advisories licenses
• cargo build --locked --workspace
• cargo shear --check-test-targets --deny-warnings --locked
• mdbook build _context/wiki
• mdbook test _context/wiki

Closes IBM/mcp-context-forge#6185