Fix local block validation in presets and defaults

[OllieinCanada]

Fixes #965

What changed

• validate section-local block types referenced by presets
• validate section-local block types referenced by default.blocks
• preserve valid references to locally declared blocks

Root cause

ValidBlockTarget returned as soon as it found a local block declaration, so preset and default references were never checked. In addition, ordinary default block entries were omitted from the collected default-level blocks unless they happened to have a name property.

The check now compares top-level preset and default block types with the section's local declarations before returning. Theme-block validation remains on its existing path.

Tests

• focused ValidBlockTarget suite: 73 passed
• full theme-check-common source suite: 4,855 passed
• tsc -b packages/theme-check-common/tsconfig.build.json
• tsc -p packages/theme-check-common/tsconfig.json --noEmit
• pnpm format:check
• git diff --check