Add type scale tokens, replace arbitrary font sizes

An h3 was 17px against a 15px body — 1.13× — so it carried a whole level of
hierarchy on weight alone and read as a bold paragraph. Below about 1.15 the eye
takes the weight change as the entire signal and stops looking for a size
change, and the extra pixels buy nothing. The document range is now 15 / 18 / 22
/ 28, and a test holds the ratio.

Rail text drops to 13px, under the document's 15. A rail set at the document's
size competes with it.

Eighteen call sites named a size the theme had never heard of, in four spellings
across three actual sizes — `text-[10px]` and `text-[0.625rem]` being the same
size written two ways. All of them are one named step now, and a test fails on
the next arbitrary size to appear. Nothing shrank: the 8px badges went up.

Line heights are declared beside each size, because Tailwind computes its own
against its own sizes and would otherwise have paired new type with the old
rhythm.

`--text-*: initial` first, so a rung we do not declare stops resolving to
Tailwind's — ours stopped at `2xl`/28px and its untouched `3xl` is 30px, two
pixels apart, in a change about steps being far enough apart to read as steps.

The document reads `--plan-body` rather than `--text-base` directly. MDXEditor
declares `--text-base: 1rem` on its own root, which sits between `:root` and
the prose, so reading the theme token inside the editor returns its value and
not ours — the body would have gone to 16px while the scale said 15. `.plan` is
outside that root, so it catches the theme's value on the way past and passes it
down under a name nothing else claims. Measured in a browser: 16px before, 15px
after.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>