docs: add architecture documentation

Explains sukr's internal design:
- Pipeline overview with Mermaid flowchart
- Module responsibilities table
- Event-based interception pattern
- Zero-JS philosophy rationale
- Static configuration and single-pass discovery patterns

docs/content/architecture.md

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+---
+title: "Architecture"
+description: "How sukr transforms markdown into zero-JS static sites"
+weight: 5
+toc: true
+---
+
+Sukr is a 12-module static site compiler. Every feature that would typically require client-side JavaScript is moved to build-time.
+
+## Pipeline Overview
+
+```mermaid
+flowchart LR
+    A[Walk content/] --> B[Parse frontmatter]
+    B --> C[Stream markdown]
+    C --> D{Code block?}
+    D -->|Yes| E[Intercept]
+    D -->|No| F[Pass through]
+    E --> G[Render HTML]
+    F --> G
+    G --> H[Apply template]
+    H --> I[Write public/]
+```
+
+## Module Responsibilities
+
+| Module               | Purpose                                             |
+| :------------------- | :-------------------------------------------------- |
+| `main.rs`            | Pipeline orchestrator — wires everything together   |
+| `config.rs`          | Loads `site.toml` configuration                     |
+| `content.rs`         | Discovers sections, pages, and navigation structure |
+| `render.rs`          | Markdown→HTML with code block interception          |
+| `highlight.rs`       | Tree-sitter syntax highlighting (14 languages)      |
+| `math.rs`            | KaTeX rendering to MathML                           |
+| `mermaid.rs`         | Mermaid diagrams to inline SVG                      |
+| `css.rs`             | CSS minification via lightningcss                   |
+| `template_engine.rs` | Tera template loading and rendering                 |
+| `feed.rs`            | Atom feed generation                                |
+| `sitemap.rs`         | XML sitemap generation                              |
+| `error.rs`           | Structured error types with source chaining         |
+
+## The Interception Pattern
+
+The core innovation is **event-based interception**. Rather than parsing markdown into an AST and walking it twice, sukr streams `pulldown-cmark` events and intercepts specific patterns:
+
+```mermaid
+flowchart TD
+    A[pulldown-cmark event stream] --> B{Event type?}
+    B -->|Start CodeBlock| C[Buffer content]
+    C --> D{Language tag?}
+    D -->|rust, python, etc.| E[Tree-sitter highlighting]
+    D -->|mermaid| F[Mermaid SVG rendering]
+    D -->|math delimiters| G[KaTeX MathML]
+    D -->|unknown| H[HTML escape only]
+    E --> I[Emit highlighted spans]
+    F --> I
+    G --> I
+    H --> I
+    B -->|Other events| J[Pass through]
+    I --> K[Continue stream]
+    J --> K
+```
+
+This pattern avoids buffering the entire document. Each code block is processed in isolation as it streams through.
+
+## Why Zero-JS
+
+Traditional SSGs ship JavaScript for:
+
+| Feature             | Typical Approach       | Sukr Approach                                     |
+| :------------------ | :--------------------- | :------------------------------------------------ |
+| Syntax highlighting | Prism.js, Highlight.js | Tree-sitter at build-time → `<span class="hl-*">` |
+| Math rendering      | MathJax, KaTeX.js      | KaTeX at build-time → MathML (browser-native)     |
+| Diagrams            | Mermaid.js             | mermaid-rs at build-time → inline SVG             |
+| Mobile nav          | JavaScript toggle      | CSS `:has()` + checkbox hack                      |
+
+The result: **zero bytes of JavaScript** in the output. Pages load instantly, work without JS enabled, and avoid the complexity of client-side hydration.
+
+## Static Configuration Pattern
+
+Tree-sitter grammars are expensive to initialize. Sukr uses `LazyLock` to create each language configuration exactly once:
+
+```rust
+static RUST_CONFIG: LazyLock<HighlightConfiguration> = LazyLock::new(|| {
+    let mut config = HighlightConfiguration::new(
+        tree_sitter_rust::LANGUAGE.into(),
+        "rust",
+        tree_sitter_rust::HIGHLIGHTS_QUERY,
+        "",
+    ).expect("query should be valid");
+    config.configure(HIGHLIGHT_NAMES);
+    config
+});
+```
+
+This pattern ensures O(1) lookup per language regardless of how many code blocks exist in the site.
+
+## Single-Pass Content Discovery
+
+The `SiteManifest` struct aggregates all content in one filesystem traversal:
+
+- Homepage (`_index.md` at root)
+- Sections (directories with `_index.md`)
+- Section items (posts, projects, pages)
+- Navigation tree (derived from structure + frontmatter weights)
+
+This avoids repeated directory scans during template rendering.