Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@docs/proposals/test-mode.md`:
- Line 46: The Markdown line "Recording source-build failures when fallback
succeeds ([`#1166`](`#limitations`))." contains a broken MyST anchor; edit that text
in docs/proposals/test-mode.md to remove the fragment by replacing the inline
link target "(`#limitations`)" with either a plain reference "[`#1166`]" or a full
GitHub issue URL "(https://github.com/.../issues/1166)" so the MyST parser can
resolve it (i.e., change "([`#1166`](`#limitations`))" to "([`#1166`])" or to the full
issue link).
- Around line 58-79: The Mermaid code fences in docs/proposals/test-mode.md are
causing Sphinx warnings because docs/conf.py only enables myst_parser
(myst_enable_extensions = ["colon_fence"]) and doesn’t treat ```mermaid as a
directive; fix by either converting/removing the mermaid diagrams in
test-mode.md to plain text/ASCII or static images, or add a Mermaid extension
and MyST config: install and enable a Sphinx Mermaid extension (e.g.,
sphinxcontrib-mermaid) and update docs/conf.py to treat mermaid fences as
directives (e.g., add myst_fence_as_directive = ["mermaid"] alongside
myst_enable_extensions) so the ```mermaid blocks are recognized and the
sphinx-build --fail-on-warning run no longer errors.