v0.3.3

Further build pipeline optimizations: eliminate redundant file walks and per-page allocations.

• Link checking folded into post-process pass: internal link validation now runs during the existing HTML post-processing walk instead of a separate file read pass
• Cached i18n context per language: the 37-key UI strings JSON object is now built once per language instead of reconstructed on every page render
• Hoisted shortcode site context: the identical sc_site JSON is pre-computed once instead of rebuilt per page during content processing
• Removed redundant string clone: expanded_body.clone() eliminated when math rendering is disabled

1k pages cold build: ~357ms → ~275ms on Apple M4 Pro (cumulative from v0.3.1: ~443ms → ~275ms, 38% faster).