feat: support axis-specific padding component sub-tokens

[zachshallbetter]

Implemented support for axis-specific padding properties (paddingX and paddingY) inside component token definitions (resolves Issue #160).

Background & Motivation

By default, the linter and spec support a uniform padding sub-token for components. However, asymmetric layouts (e.g., text inputs with different vertical and horizontal offsets) are standard in production design systems. Adding paddingX and paddingY addresses this capability gap natively.

• This matches common framework layout models (Tailwind's px/py, SwiftUI, Flutter) and avoids nested layout mess (such as the schema formats discussed in #Issue Feature: Add Support for Component Variations #17 ).
• This aligns with the philosophy of PR feat(spec): add 9 standard CSS component sub-tokens #89 (avoiding CSS property bloat like paddingLeft/paddingTop) by introducing structural layout minimums rather than exhaustive box-model properties.
• Sourced independently from PR feat: support physical dimension units (pt, mm, cm, in) for print/PDF design systems #164 to prevent merge conflicts.

Changes

• Configuration:
  • Add paddingX and paddingY to the recognized component_sub_tokens schema list in spec-config.yaml.
• Test Coverage:
  • Add validation tests in broken-ref.test.ts verifying that component definitions containing paddingX and paddingY do not emit unrecognized sub-token warnings.
• Documentation:
  • Update the valid component properties list in README.md.
  • Regenerate docs/spec.md using the spec compiler.

[zachshallbetter]

Ignore this mess. I merged from main instead of the feat branch. *Oops.

See PR #166