## Why
Inline visualization viewer documents must remain outside locations that a
sandboxed session can modify before they are opened in a browser.
## What changed
- Materialize viewer documents in a dedicated cache under `CODEX_HOME`, keyed
by the source and artifact thread IDs, instead of alongside visualization
artifacts.
- Create visualization links only when the active filesystem policy cannot
write to the viewer cache. This also disables links for full-disk-write
sessions and applies the policy consistently when loading transcript history.
- Reject viewer cache paths containing symbolic links and track materialized
documents in memory so unchanged viewers are reused without trusting existing
file contents.
## Testing
- Verify that rendering an unchanged visualization reuses the existing viewer
file, while an updated fragment refreshes it.
GitOrigin-RevId: 12b7fbe522a68076e5d683a987b7801940d131c2
902bd9e06b