feat(executor): add direct block execution for conditional blocks

[atinseau]

Summary

• Add tryDirectBlockExecution() to evaluate #if/#unless blocks directly without Handlebars rendering
• Preserves non-string return types (e.g. arrays from map) when the selected branch resolves to a non-primitive value
• Falls back to existing Handlebars rendering path when direct execution is not applicable

Why

Previously, wrapping a map call inside {{#if condition}}...{{/if}} would cause Handlebars to stringify the array result. Direct block execution evaluates the condition and delegates to the existing type-preserving execution paths, so callers receive the raw array (or other typed value) instead of a string.

Test plan

• bun test tests/direct-block-execution.spec.ts — all 557-line suite passes
• Verify #if with truthy/falsy conditions returns correct branch value
• Verify #unless inverts condition correctly
• Verify fallback to Handlebars when branch is a plain string mustache
• Verify nested conditionals work correctly