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/lexer.rs

@@ -493,6 +493,8 @@ impl<'a> Lexer<'a> {
                                 Some('"') => '"',
                                 Some('0') => '\0',
                                 Some('\'') => '\'',
+                                Some('{') => '{',
+                                Some('}') => '}',
                                 Some('x') => {
                                     // Hex escape \xNN
                                     let h1 = self.advance().and_then(|c| c.to_digit(16));