docs(design): mark direction D as built

[sunerpy]

Summary

A docs-only follow-up to #298 and #300, acting on the final goal review's one note.

docs/design/viewer-d.md still said "Design draft … No UI code exists yet", and its §13 listed the light variant as an open question.

• Status now says the viewer is built to these numbers since feat(ui): direction D design and a light theme #300, and points at the deviation log in docs/ui.md ("Direction D as built").
• §13 Light variant now records that §3.6 settled it: Daylight, following prefers-color-scheme until the reader picks System, Dark or Light.
• §13 Ligatures now records that the build turned them off for source fidelity, logged for the owner to review.

The rest of the spec is unchanged.

Verification

make docs-check and make fmt-check pass. The file is in .oxfmtignore, so it is not reformatted. There is no code change.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@           Coverage Diff           @@
##             main     #301   +/-   ##
=======================================
  Coverage   95.05%   95.05%           
=======================================
  Files         197      197           
  Lines      108387   108387           
=======================================
+ Hits       103023   103025    +2     
+ Misses       5364     5362    -2     

see 1 file with indirect coverage changes

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