Replace container gap with gapX and gapY

[han4wluc]

Summary

• remove container support for the legacy gap field and expose only gapX / gapY
• update parser semantics, schemas, docs, templates, VT specs, and parser fixtures to the new spacing contract
• add parser coverage for asymmetric axis spacing and an explicit error when gap is provided

Verification

• bun run vitest run spec/parser/parseContainer.test.yaml
• bun run test
• bun run build
• pre-push hook reran bunx prettier --check src and vitest run