feat(docgen): deep-merge user-supplied JSON-LD frame with built-in default

[ddeboer]

Closes #313.

Summary

• A user-supplied JSON-LD frame is now deep-merged on top of docgen’s built-in default, so consumers only need to specify their additions (e.g. extra @context entries) instead of duplicating all of docgen’s defaults.
• Plain objects merge key-by-key with user values winning; arrays and primitives in the user frame replace the default. Built-in coercions can still be overridden by redefining the same key.
• The default frame path is now resolved internally by frame(), so generateDocumentation() and the CLI’s -f option both treat the user frame as optional.

A consumer’s frame can now be as small as:

{
  "@context": {
    "nde": "https://def.nde.nl#",
    "nde:futureChange": {},
    "nde:version": {}
  }
}

Test plan

• npx nx run @lde/docgen:test – existing integration test now exercises the no-frame default path; new frame.test.ts covers default-only, partial merge (preserves default coercions while adding user keys), and override (user coercion replaces default)
• npx nx run @lde/docgen:typecheck and :build pass
• 100% line/branch coverage on frame.ts