Migrate the package build to tsdown

One tool replaces four: both vite lib configs, vite-plugin-dts,
api-extractor and the terser postbuild. tsdown covers each of their jobs
natively — per-entry declaration bundling (from the actual lib entry, so
the vite-plugin-dts types-field footgun is structurally gone), oxc
minification, and CSS extraction with @import inlining via @tsdown/css.

The config keeps the two-isolated-builds decision as a config array: a
single multi-entry build would still split the shared popover primitive
into a common chunk and merge the stylesheets. Filenames, the exports
map and the CDN globals (window["driver"]["js"].driver, driverHints)
are unchanged; attw/publint stay green across node10/node16/bundler for
both entries.

Verified against the previous pipeline's output: identical file list,
identical driver.css rule set, hints.css differs only in Lightning CSS
keeping the inset shorthand our source already uses (baseline everywhere
driver.js supports), and every artifact's gzip size is equal or smaller.
Build drops from ~6.8s to ~2.2s — three TypeScript passes down to one
plus tsdown's per-entry declaration emit.

The postbuild shrinks to the two plain .d.ts copies node10 resolution
needs. vitest config moves to vitest.config.ts (vite itself is no longer
a direct dependency), and a local *.css module declaration replaces
vite/client's ambient types.