Archive responsive graph viewport plan

[ktjn]

Summary

• move the completed responsive graph viewport plan and design into their archive directories
• update the reciprocal plan/design links for their archived locations
• repair references from the archived visualization and graph-theme designs

Why

PR #237 merged the implementation. Repository policy keeps only active or unstarted work in the top-level plans/ and specs/ directories, so the completed documents now belong in the archive.

Impact

This is documentation bookkeeping only; runtime behavior is unchanged.

Validation

• uv run ruff format .
• uv run ruff check .
• mypy baseline ratchet — passed with 404 current errors and 45 resolved baseline errors
• uv run pytest --tb=short — 1531 passed, 15 skipped
• local Markdown structure, fence, placeholder, and cross-reference checks passed

Doc/spec review: all phases passed