fix(ui): let a classed label own its box instead of the generic field-label rule

[argszero]

Summary

.form label { display: flex; flex-direction: column; … } was unconditional. At specificity (0,1,1) it outranked the component layer (0,1,0) for every <label> carrying a component class, so the box the class asked for never applied — lower specificity loses regardless of source order.

Two shapes are hit, from two different origins but one root cause:

Victim	Symptom	Origin
.check-line	The Settings → Notifications checkboxes and the Preferences → table-density radios render stacked: the control sits above its caption, centred.	#157 (09e4121) renamed the markup from class="check" to the prototype-canonical .check-line that #154 (75dcb2c) introduced — but left the stylesheet's .form label.check (0,2,1) behind. That rule was the only thing turning a checkbox row back to a row, and it stopped matching anything.
.chip	The share-form weekday chips render as circles instead of pills.	#32 (d31641d) turned the chips from <button> into <label>; the display:inline-flex #154 gave them was overridden by the generic rule.

.checkbox-line (login "remember me") only survived because #61 added flex-direction: row !important; ironically the prototype-canonical .check-line has no such protection.

Related Issue

Changes

• ui/css/style.css: guard the two generic field-label rules with :not([class]) — a classed label takes its box from its own class, and the generic rule now serves only unclassed field labels.
• ui/css/style.css: drop the now-dead .density-options .check rule (feat(ui): redesign settings/admin/ops views on the landed component layer #157 renamed that markup too) and give .checkbox, .checkbox-line back their own display: flex, which they used to borrow from the generic rule.
• src/layout_gate.rs (new, #[cfg(test)], zero new dependencies): two derived rules so this cannot silently recur.
• src/main.rs: register the gate (#[cfg(test)] mod layout_gate;).
• 涉及配置/数据结构的改动已同步示例文件 — n/a

The gate

Not the two classes, but the shape they share:

• R1 (roster, derived from ui/index.html) — every class used on a <label> must have a stylesheet rule that targets it and declares display. A label's box must come from its class, never borrowed from a generic rule.
• R2 (mechanism) — any rule whose rightmost compound selector is a bare label and that declares display / flex-direction must carry a guard excluding classed elements (:not([class])).

Together they are the invariant: a generic field-label rule serves only labels without a class.

Expected values are entirely derived (roster from ui/index.html, rules from ui/css/style.css), so no snapshot is written and the next component class is covered on arrival.

Rejected alternatives (each only plugs today's two holes): add !important to .check-line; add a .form label.X row override per victim; only delete the dead .form label.check.

Tests

• cargo test — 435 passed / 0 failed (baseline 429; +6 from the new gate)
• cargo fmt --check — clean
• cargo clippy --all-targets -- -D warnings — clean
• New unit tests added

A/B (same tree, only ui/css/style.css swapped for upstream/main's):

pre-fix : R1 violations = ["checkbox"]
          R2 violations = [".form label { … }", ".login-form label { … }"]
          → axis test FAILED, positive control FAILED (guarded-rule set empty)
fixed   : R1 violations = []   R2 violations = []   → 6/6 ok

Second instrument — jsdom, computed style per shape (whole document, both trees):

shape	n	before	after
.form label.check-line	5	flex / column	flex / row
label.chip	8	flex / column	inline-flex / row
label.checkbox	1	flex / row	flex / row
label.checkbox-line	1	flex / row	flex / row
.form label:not([class])	23	flex / column	flex / column
.login-form label:not([class])	12	flex / column	flex / column

The fix changes exactly the classed labels; the unclassed field labels are untouched in both trees.

Scope (stated honestly): the gate is lexical — it proves who is allowed to give a label its box, not the painted layout. That half is covered by the jsdom probe above and by headless-Chrome before/after screenshots.

Checklist

• 分支命名符合约定 (fix/…)
• Commit message 使用 Conventional Commits 格式
• 单一职责，改动最小化

[argszero]

✅ Self-review (committer, allow_self_merge). CI: msrv pass, test / fmt / clippy pass on 36654784704; mergeStateStatus: CLEAN.

Checked:

1. The rule change is the minimal shape of the fix. :not([class]) on the two generic field-label rules is what lets the component layer win; the alternative (!important per victim, or a .form label.X override per victim) only plugs the two holes that exist today. R1 of the new gate covers the next component class on arrival.
2. Nothing else regressed. The jsdom sweep shows the unclassed field labels (.form ×23, .login-form ×12) are column in both trees, and label.checkbox / label.checkbox-line stay row — the latter two used to borrow display:flex from the generic rule, which the patch now grants them explicitly, so no consumer lost a box.
3. The dead rule really is dead. .density-options .check had zero matching elements after feat(ui): redesign settings/admin/ops views on the landed component layer #157 renamed the markup; git grep finds no class="check" anywhere in ui/.
4. The gate is derived, not a snapshot. Roster comes from ui/index.html, rules from ui/css/style.css; the positive control asserts both scans see something (≥4 classes, ≥2 guarded rules), so neither rule can pass vacuously on an empty set.
5. Gate teeth, measured. Swapping only ui/css/style.css for upstream/main's reproduces R1 = ["checkbox"], R2 = [".form label …", ".login-form label …"]; restoring it gives 6/6.
6. Comment-safe scanners. The stylesheet contains explanatory comments that quote selectors verbatim, so the gate masks comments and string literals with an equal-length mask before parsing — and a test asserts the masked corpus yields exactly the two real rules.

Scope, stated honestly: the gate is lexical. It proves who may give a label its box, not what the browser paints. That half is the jsdom table in the description plus the headless-Chrome screenshots taken during recon.