fix: write D9Skeleton's margin classes in full and rebuild lines on change (BITS-212)

[PhilReinking]

D9Skeleton now writes its eight line margin classes out in full, so Input's Tailwind generates them and the loader lines get varied widths again. On main, Input's CSS has only mr-4 and mr-6, so most lines render full width. The lines are also a computed now: lines of -1 or 2.5 no longer throw, and a new lines value re-renders.

Fixes BITS-212

• Fractional lines round down, negative ones render none. The line block hides when the count rounds to 0, so 0.5 leaves no empty gap.
• Each line still gets a random width. A lines change re-rolls all widths, not only the new lines.
• Props and slots are unchanged, so Input needs no change.

Tested: check, build and tests pass, and the new spec fails on main. Input's Tailwind build has all 8 classes with this branch. Storybook checked in Brave.

Not tested: Input's dashboard itself in a browser, and Chrome, Safari or Firefox by hand.

🤖 Generated with Claude Code

[PhilReinking]

OK at 87b7ac6, CI green.

Checked against main: Input's own Tailwind (3.4.19) on this build emits all 8 .ipt .mr-N rules, on main only mr-4 and mr-6. The 3 new specs fail on main's Skeleton.vue. Widths stay the same on unrelated re-renders and re-roll only when lines changes. NaN, null and 0.5 render no lines and no gap; Infinity still throws, as on main.