feat(checkbox): MoonSize tiers, the reviewed checkbox design and bundled Inter

[ThusMad]

What & why

Three commits, each passing the guardrails on its own:

1. feat(fonts): bundle Inter and Geist Mono so apps render them without installing. The theme asks for "Inter" and "Geist Mono", but only the web platform registered the bundled files. On a desktop without Inter installed, every MoonUI app fell back to the system face (Segoe UI on Windows), which has no Medium weight and isn't monospaced. The bundled Inter-SemiBold.ttf was also a byte-for-byte copy of Inter-Regular.ttf. moon_ui_components::init now registers the fonts, embedded as 'static bytes, with the app's text system. Inter ships at Regular, Medium, SemiBold and Bold: the 18pt optical-size static cut, with the family renamed from "Inter 18pt" to "Inter" and STAT removed, so the theme's family resolves on DirectWrite, CoreText and cosmic-text without an alias. NOTICE records the OFL licence and the modification. Binary size grows by about 1.7 MB.

2. fix(root): clicking empty space moves focus off the focused control. A press only moved focus when it landed on a focusable element, so clicking empty space left the last control focused indefinitely. A window with nothing focused also never delivered Tab to Root's handler. Root now tracks its own focus handle (not a tab stop). A click that no control claims focuses Root: the previous control blurs, and Tab still works.

3. feat(checkbox): MoonSize tiers and the reviewed checkbox design.

• Shared size scale. MoonSize { Xs, Sm, Md, Lg, Xl, Xxl } reuses the previously unused foundation enum; the old variant names stay as deprecated consts and serde aliases. A component renders the tiers it supports and snaps the rest to the nearest one.
• Checkbox API. Checkboxes support Sm and Md. size(impl Into<MoonCheckboxSize>) accepts a tier directly, and Compact and Normal remain as deprecated aliases.

	sm	md
Box / radius	16 / 4px	20 / 6px
Mark / stroke	12 / 1.67px	14 / 2px
Text (label Medium, description Regular)	14/20	16/24
Box→text / label→description gap	8 / 0px	12 / 2px

• Checked state. The box is one solid tone, border and fill alike, and the mark uses the palette ink with the best contrast on that tone.
• Marks. The check and indeterminate minus come from SVG assets. Their stroke-width is rewritten at paint time and the colour is applied as a tint, so the shape stays a file while stroke and colour are metrics. The mark is now centred inside the 1px border; it previously sat 1px low and right.
• description() support text. Appears under the label. With a description, the box centres on the label's line.
• Scaling. The tiers follow only UI zoom; custom sizes keep following text scaling.
• Interaction. Pointer cursor, no hover or press fill. A 2px focus ring whose outer edge sits 4px outside the box is drawn as an overlay, so it never changes layout. Pressing a checkbox now focuses it.
• Audit contract. checkbox.checked_glyph.asset moves from a golden-PNG presence check to the stroke-rewrite and mark-geometry tests.
• Baselines. The API baseline records the additions and declares the two widened signatures (MoonCheckbox::size, and the foundation re-export gaining MoonSize). The mirror and audit baselines record the reviewed base-checkbox drift, which is still one file and within budget. .vscode is ignored.

How to verify

• tools\run-component-guardrails.ps1 passes on every commit: 454, 455 and 464 component tests, and 7 gallery tests each time.
• cargo test -p moon-gpui passes (186 tests).
• New tests:
  • tests/bundled_fonts.rs runs the real platform (headless Windows uses a no-op text system) without the libtest harness, because quitting that platform ends the process. It checks that Inter 400/500/600/700 and Geist Mono resolve to distinct faces. It fails when registration is removed. CI note: it creates DirectX devices on windows-latest; GPUI enumerates the software adapter there, but this has not run in CI yet.
  • root::clicking_empty_space_moves_focus_off_control fails when track_focus is removed.
  • The checkbox layout tests use debug-selector probes: mark size and centring, description stacking and gaps, focus ring geometry without layout movement, and click-to-focus followed by Tab. They also cover the stroke rewrite for both icons, tier snapping, deprecated aliases, and serde reading the old MoonSize names.

Not in this PR: refreshed gallery golden snapshots. The new checkbox geometry and the bundled Inter change most pages, and the opt-in snapshot comparison needs a reviewed capture.

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