refactor(ui): measured Ribbon layout with priority-prefix overflow

[Limdongcheng]

Summary

• Ribbon group and tile widths now come from real galley measurement through an injected measurer (headless-testable), replacing the per-character estimates that drift on CJK and long labels; the More overflow reservation is measured the same way instead of a fixed 86 px.
• The overflow fill admits groups by priority and stops at the first group that does not fit, so the visible set is always the highest-priority prefix. Previously a wide high-priority group could land in More while narrower low-priority groups stayed visible.
• Every (tab, group) pair has an explicit left-to-right order, guarded by an exhaustive test over the placement tables; roughly a third of the groups used to tie at u8::MAX and ordered by catalog-iteration accident (Data ▸ Export, Process ▸ Processing, Analyze ▸ Statistics, the Figure/Arrange property groups, …).
• The screenshot harness gains Op::RibbonTab and captures every task tab at 900 px, so Ribbon changes have visual coverage beyond the Analyze tab.
• ribbon.rs splits into ribbon/layout.rs + ribbon/buttons.rs, and the command roster moves to commands/roster.rs, keeping every file under the 800-line limit.

No intended visual change at today's widths apart from group order becoming deliberate.

Validation

• cargo pr-check passes (fmt, file size, dependency policy, both frontends, clippy with denied warnings, tests).
• New unit tests: group-order exhaustiveness over the placement tables, the overflow prefix property, and measured-density selection.
• PLOTX_SHOT before/after at 720/900/1440 px in both themes.

UI display logic

• New or changed actions go through the command catalog (CommandId +
  describe) and are searchable in the command palette. (No actions added;
  the roster extraction is code motion.)
• Hiding happens only at Ribbon-group level (dataset kind or width
  budget); transient states disable with a disabled_reason that says how
  to unblock.
• No layout changes from background events; panels are never auto-closed.
• New panels or empty regions show an empty state with a next step. (None
  added.)
• Docs updated in docs/ (English and zh-CN) for user-visible behavior.
  (No documented behavior changes in this PR.)

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