feat: add blu-site static site generator and fix language issues

Build a complete static site generator in Lux that faithfully clones
blu.cx (elmstatic). Generates 14 post pages, section indexes, tag pages,
and a home page with snippets grid from markdown content.

Language fixes discovered during development:
- Add \{ and \} escape sequences in string literals (lexer)
- Register String.indexOf and String.lastIndexOf in type checker
- Fix formatter to preserve brace escapes in string literals
- Improve LSP hover to show documentation for let bindings and functions

ISSUES.md documents 15 Lux language limitations found during the project.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/formatter.rs

@@ -728,7 +728,7 @@ impl Formatter {
         match &lit.kind {
             LiteralKind::Int(n) => n.to_string(),
             LiteralKind::Float(f) => format!("{}", f),
-            LiteralKind::String(s) => format!("\"{}\"", s.replace('\\', "\\\\").replace('"', "\\\"")),
+            LiteralKind::String(s) => format!("\"{}\"", s.replace('\\', "\\\\").replace('"', "\\\"").replace('{', "\\{").replace('}', "\\}")),
             LiteralKind::Char(c) => format!("'{}'", c),
             LiteralKind::Bool(b) => b.to_string(),
             LiteralKind::Unit => "()".to_string(),