Reintroduce bundle-size tracking with size-limit

[dogmar]

The size-limit fixtures under package/size-limit/ were removed in #9 because nothing invoked them: there was no size-limit config key, no script or CI step ran the tool, and one fixture imported ../site/, a path that predates the monorepo. The four devDependencies (size-limit, @size-limit/esbuild, @size-limit/esbuild-why, @size-limit/file) were removed with them.

To bring bundle-size tracking back properly:

• Recover the entry fixtures from git history (git show <pre-#9>:package/size-limit/) and fix the stale ../site/ import (now ../../website/src/lib/drag-components)
• Add a size-limit config (package.json key or .size-limit.json) with budgets per entry (full bundle, unstyled, unstyled-drag, unstyled-mini)
• Wire it into CI after the library build so regressions fail the PR (fixtures import ../dist/index.js, so a vp run @klinking/colander#build must precede it)
• Exclude the fixtures from vp lint type checking or give them a tsconfig that resolves ../dist (this was the reason they broke type checking in docs: precompile Markdoc into React route components at build time #9)

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