[Accessibility] Expand markup accessibility diagnostics

[cssbruno]

Source: split follow-up from #237 after the #61 routing/layout/view baseline.

Scope:

• Expand accessibility diagnostics beyond the implemented missing_img_alt warning.
• Evaluate labels, empty links, button type, and heading-order diagnostics for the current view {} AST and generated-output model.
• Keep diagnostics as compiler warnings unless a specific failure is security-sensitive or breaks generated behavior.
• Avoid introducing browser/runtime checks when the compiler can identify the issue from source markup.

Acceptance criteria:

• New diagnostic codes are registered in internal/diagnostics/registry.go, documented in docs/reference/diagnostic-codes.md, and exposed through gowdk explain when useful.
• docs/language/markup.md and docs/product/requirements.md state implemented and still-planned accessibility coverage.
• Tests cover pages, components, and layouts where each diagnostic applies, including source spans and check --json severity.
• Existing missing_img_alt behavior remains stable.

Related: #61, #237.