docs: garden repository notes

[t-kalinowski]

Summary

This is a draft docs-upkeep PR for curating stale notes, futurework, and documentation pointers.

• Records the current decision on the R custom graphics device investigation: a custom device is not currently recommended, because the known plot/stdout ordering bug was fixed in server-side timeline reconstruction and a device would not remove the need for that ordering logic.

Diff Composition

• Docs: updates one futurework note and its docs index entry.
• Runtime code: none.
• Tests, inline tests, snapshots: none.

Public Facing Changes

No runtime or public API changes. This is documentation only.

Internal-Only Changes

Clarifies futurework guidance so stale notes do not point agents toward outdated implementation directions.

Testing

• cargo test --test docs_contracts
• git diff --check