docs(webkit): add the one-content-column principle to webkit-ui-craft

[gabriel-lisboa-azion]

Building the console rail (#853 / #854) surfaced a failure mode the skill had no words for, so it kept happening.

The principle

One content column. Inside a vertical list — a nav rail, a menu, a settings list, any stack of rows — every row's content starts on the same x, whatever the row is. A section title's text, a row's leading glyph, and a row with no glyph at all all begin on that column. A row that reserves a glyph box pads by the column minus the glyph's own centring, so the glyph lands on it rather than the box. Nesting shifts the whole column by exactly one indent step, so the alignment cascades at every depth instead of being re-derived per level.

Three bugs follow from breaking it — each one actually happened

• Never reserve an empty glyph box. A box with no glyph misreports where the row's content starts: the label sits off the column its siblings hold, and anything drawn from that content (a tree rail, a hover surface) anchors to blank space. This is the exact bug fixed in menu-item in feat(webkit): add the Menu compound and move MenuItem into it #853.
• Derive the indent from tokens, never a literal. Hold the step and the column in two custom properties and compute the rest, so an elbow or rail cannot come unstuck from the rows it connects. Avoid any --spacing-* token redefined at a breakpoint — the column would drift as the viewport grows.
• Structure lines live in the gutter, never over a row. A tree rail or elbow stops at the row's box edge, not at its text. The row's hover and selected surfaces fill that box, so a line drawn further is painted underneath them and reads as a glitch on exactly the states a user interacts with.

And: verify it by measuring, not by looking. Read the rendered x of each row type and assert they are equal; assert a rail's right edge never exceeds the row surface's left edge. A 4px break is invisible in review and obvious in production.

Why this is its own PR

git-workflow.md keeps shared docs out of code PRs, and this is a shipped skill (packages/webkit/cli-templates/) — it travels to consuming projects, so it is a public interface like a component. It slots in as principle 5 and renumbers the three that follow.

check-authoring-docs ratchet: 7 known violations, 0 new.

[CLAassistant]

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