feat!: replace YAML frontmatter with TOML

Replace hand-rolled YAML parser (70 lines) with serde-backed
toml::from_str<Frontmatter> (6 lines). Frontmatter delimiter
changes from --- to +++ (Hugo TOML convention).

New Frontmatter fields:
- draft: bool (#[serde(default)]) for draft filtering
- aliases: Vec<String> (#[serde(default)]) for URL redirects

Date field upgraded from Option<String> to Option<NaiveDate>
with custom deserializer for TOML native dates. Parse, don't
validate: invalid dates now fail at deserialization time.

Add chrono dependency with serde feature. Update cascade in
sitemap.rs (NaiveDate→String at boundary), template_engine.rs
(FrontmatterContext gains draft/aliases), and all 14 tests.

BREAKING CHANGE: Content files must use +++ TOML frontmatter
instead of --- YAML frontmatter.

Cargo.toml

@@ -64,9 +64,10 @@ lightningcss = "1.0.0-alpha.70"
 # tera: Jinja2-style templates, runtime-loaded (no recompilation needed).
 # serde/toml: Configuration parsing. Standard choices, no lighter alternatives.
 # ─────────────────────────────────────────────────────────────────────────────
-serde = { version = "1", features = ["derive"] }
-tera  = "1"
-toml  = "0.8"
+chrono = { version = "0.4", default-features = false, features = ["serde"] }
+serde  = { version = "1", features = ["derive"] }
+tera   = "1"
+toml   = "0.8"
 
 # ─────────────────────────────────────────────────────────────────────────────
 # Patches