feat(content): add filesystem-driven nav discovery

Add NavItem struct and discover_nav() function to scan content
directory and automatically build navigation from:
- Top-level .md files (pages)
- Directories with _index.md (sections)

Navigation is sorted by frontmatter weight, then alphabetically.
Custom nav_label field allows overriding title in nav menu.

Includes 5 unit tests covering page/section discovery, weight
ordering, and nav_label support.

Cargo.lock

@@ -385,6 +385,16 @@ version = "1.0.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "877a4ace8713b0bcf2a4e7eec82529c029f1d0619886d18145fea96c3ffe5c0f"
 
+[[package]]
+name = "errno"
+version = "0.3.14"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "39cab71617ae0d63f51a36d69f866391735b51691dbda63cf6f96d042b63efeb"
+dependencies = [
+ "libc",
+ "windows-sys",
+]
+
 [[package]]
 name = "fastrand"
 version = "2.3.0"
@@ -630,6 +640,12 @@ dependencies = [
  "syn 1.0.109",
 ]
 
+[[package]]
+name = "linux-raw-sys"
+version = "0.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "df1d3c3b53da64cf5760482273a98e575c651a67eec7f77df96b5b642de8f039"
+
 [[package]]
 name = "lock_api"
 version = "0.4.14"
@@ -707,6 +723,7 @@ dependencies = [
  "mermaid-rs-renderer",
  "pulldown-cmark",
  "serde",
+ "tempfile",
  "thiserror 2.0.18",
  "toml 0.8.23",
  "tree-sitter",
@@ -1181,6 +1198,19 @@ version = "2.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "357703d41365b4b27c590e3ed91eabb1b663f07c4c084095e60cbed4362dff0d"
 
+[[package]]
+name = "rustix"
+version = "1.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "146c9e247ccc180c1f61615433868c99f3de3ae256a30a43b49f67c2d9171f34"
+dependencies = [
+ "bitflags",
+ "errno",
+ "libc",
+ "linux-raw-sys",
+ "windows-sys",
+]
+
 [[package]]
 name = "rustversion"
 version = "1.0.22"
@@ -1375,6 +1405,19 @@ version = "1.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "55937e1799185b12863d447f42597ed69d9928686b8d88a1df17376a097d8369"
 
+[[package]]
+name = "tempfile"
+version = "3.24.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "655da9c7eb6305c55742045d5a8d2037996d61d8de95806335c7c86ce0f82e9c"
+dependencies = [
+ "fastrand",
+ "getrandom 0.3.4",
+ "once_cell",
+ "rustix",
+ "windows-sys",
+]
+
 [[package]]
 name = "thiserror"
 version = "1.0.69"