Bind editor text to explicit typography roles

[librowski]

SDK text still depended on deprecated size-only classes and a document-wide * font reset. This stacked change makes editor typography explicit without changing host-page elements outside Workflow Builder.

Stacked on #97.

What changed

• Replaced legacy SDK ax-public-h* / ax-public-p* usage and ad hoc declarations with semantic wb-text-* roles.
• Removed the document-wide font-family reset. --wb-font-family remains available for consumer-owned surfaces that opt into it, but overriding it no longer rethemes built-in text.
• Gave deprecated UI typography classes an explicit Poppins family so remaining primitives stay consistent during migration.
• Assigned the DatePicker portal its own body role instead of relying on document inheritance.
• Kept Ace monospaced for fixed-width cursor correctness while inheriting the wb-text-code size, weight, and line-height.
• Added an SDK major / UI minor changeset and updated the theming and typography guides.

Verification

• pnpm build:lib
• pnpm lint and pnpm lint:styles
• pnpm -r test
• Workspace typechecks through UI, SDK, demo, backend, worker, and AI Studio
• pnpm build:docs (212 pages)
• pnpm build
• Prettier, git diff --check, Changesets status, namespace scans, and independent review with zero findings
• Headless browser smoke at desktop and 390px mobile with external Google Fonts blocked: host body stayed unthemed, editor/modal text resolved to Poppins, Ace to monospace, and the DatePicker portal to Poppins

Known baseline

The root pnpm typecheck reaches four existing Astro diagnostics in unchanged head.astro and sidebar.astro. They reproduce identically on parent #97; the docs production build and CI docs gate pass.