feat(import-design): validate DTCG output and type font-family tokens

[shipyard-local]

Follow-up to the W3C DTCG token emitter (#6390). Adds conformance validation and accurate string typing — emitter + test only, so the importers and imported-design render/codegen flows are untouched.

What changed

• validate_dtcg() — a dependency-free W3C Design Tokens conformance check (standard $type set, value/type shape, reserved $-keys, $extensions well-formedness), run over emitter output in tests and proven against known-bad documents.
• Font-family typing — confident font-family string tokens now emit $type:"fontFamily" (comma stacks as the DTCG array form); the non-standard $type:"string" is gone.
• No drops — every other string is parked losslessly under the document-root $extensions["dev.pulp.nonStandardTokens"].

Non-breaking guarantee

Changes live only in design_tokens_w3c.{hpp,cpp} + its test + docs/skill. IRTokens, the parser, codegen, and the runtime theme are unchanged.

Verification

• 22 DTCG cases / 78 assertions pass (incl. 4 known-bad docs the validator correctly rejects).
• 476/476 token|import|codegen|theme|designmd regression tests pass — zero breakage.
• Real emit (alpha/DESIGN.md): font families → fontFamily; 63 non-standard strings parked in $extensions, none dropped.

🤖 Generated with Claude Code

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_{stamped 2026-07-19 22:42 UTC}