Published v12 declarations fail strict consumer type-checking

[bcomnes]

Problem

The published 12.0.0-beta.2 tarball does not type-check in a clean NodeNext consumer with skipLibCheck: false.
The failure reproduces under TypeScript 5.9 and TypeScript 6.0.

DOMStack-owned declaration errors include:

lib/build-pages/page-builders/html/index.d.ts: Cannot find name 'T'
lib/build-pages/page-builders/md/index.d.ts: Cannot find name 'T'

The generated htmlBuilder and mdBuilder return types reference T without declaring it.
The public declaration graph also exposes declaration errors from cpx2, and TypeScript 5.9 reports an additional Markdown declaration incompatibility.

The repository's skipLibCheck: true setting masks these failures in the current suite.

Proposed fix

• Correct or remove the leaked concrete-builder generic.
• Avoid exposing implementation-only dependency types through the public declaration graph where practical.
• Add a release test that runs npm pack, installs the tarball in a clean fixture, and runs tsc with NodeNext resolution and skipLibCheck: false.

Acceptance criteria

• Root imports from @domstack/static compile in a clean consumer.
• Public type imports from @domstack/static/types.js compile in a clean consumer.
• The packed artifact is tested rather than only the source checkout.