Working on 012

Read UX, extending and testing in docs/contributing/ before changing code.

No broken windows

Noise that stays becomes invisible, and then real problems hide in it.

• make check passes before every push: gofmt, vet, make lint (staticcheck in every module, doclint, doccheck, cognitive complexity at most 25, Go files at most 500 lines), unit tests, the excelize oracle and the libghostty e2e tests. A red check is fixed first, not worked around. Run it once, at the end, not repeatedly while iterating: make quick checks just what changed (testing).
• A warning is fixed where it points, or turned off in its tool's config with the reason written next to it (staticcheck.conf). Nothing is left standing because "it's always been there".
• Docs and comments describe the code as it is and why. How it got there (what changed, what it did before, before and after numbers, which commit or merge, who did it) belongs in commit messages; doclint enforces this. Saying why the code stays compatible with a planned change is fine: that explains the code today. Plans go in ROADMAP.md.
• Each doc has one job and one home per fact: change the section a reader would look in, link to it from elsewhere, and restructure when appending would make a wall of text. Shipped roadmap items move to its Shipped list as one line.
• Diagrams are Mermaid code blocks, never ASCII art. Draw one where a flow, a state machine or a sequence would otherwise take paragraphs (the docs site). 012's screen is a picture, a still drawn from a golden screen, never text in a code block (Pictures of 012); code blocks are for what a reader types or a shell prints.
• No scratch files, debug tests, temporary samples or build outputs (*.test, binaries) in commits; make lint refuses compiled files and anything over 2 MB.
• Visual changes are reviewed in the golden gallery (make screens).