fix(rust): replace .expect("TODO") in parser.rs (1 prod + 6 tests)

7 .expect("TODO: handle error") sites in src/parser.rs cleared.

Prod (1 site, line 453 in is_thematic_break):
  trimmed.chars().next() is statically Some because
  trimmed.len() >= 3 is guarded immediately above. Documented
  invariant: "trimmed.len() >= 3 was guarded immediately above,
  so chars().next() is always Some". SPARK proof candidate.

Tests (6 sites): all parse(...) calls inside #[cfg(test)] mod
revert to .unwrap() — panicking on Err is the test verdict.

cargo test --lib: 11/11 green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

src/parser.rs

@@ -450,7 +450,8 @@ fn is_thematic_break(line: &str) -> bool {
     if trimmed.len() < 3 {
         return false;
     }
-    let first = trimmed.chars().next().expect("TODO: handle error");
+    let first = trimmed.chars().next()
+        .expect("trimmed.len() >= 3 was guarded immediately above, so chars().next() is always Some");
     (first == '-' || first == '*' || first == '_') && trimmed.chars().all(|c| c == first)
 }
 
@@ -496,21 +497,21 @@ mod tests {
 
     #[test]
     fn parse_empty_document() {
-        let doc = parse("").expect("TODO: handle error");
+        let doc = parse("").unwrap();
         assert!(doc.title.is_none());
         assert!(doc.blocks.is_empty());
     }
 
     #[test]
     fn parse_heading_and_paragraph() {
-        let doc = parse("# Title\n\nSome text here.").expect("TODO: handle error");
+        let doc = parse("# Title\n\nSome text here.").unwrap();
         assert_eq!(doc.title.as_deref(), Some("Title"));
         assert_eq!(doc.blocks.len(), 2);
     }
 
     #[test]
     fn parse_directive() {
-        let doc = parse("@version 1.0").expect("TODO: handle error");
+        let doc = parse("@version 1.0").unwrap();
         assert_eq!(doc.directives.len(), 1);
         assert_eq!(doc.directives[0].name, "version");
         assert_eq!(doc.directives[0].value, "1.0");
@@ -519,7 +520,7 @@ mod tests {
     #[test]
     fn parse_attestation_block() {
         let input = "!attest identity=Alice role=author trust=reviewed";
-        let doc = parse(input).expect("TODO: handle error");
+        let doc = parse(input).unwrap();
         assert_eq!(doc.attestations.len(), 1);
         assert_eq!(doc.attestations[0].identity, "Alice");
         assert_eq!(doc.attestations[0].trust_level, TrustLevel::Reviewed);
@@ -528,7 +529,7 @@ mod tests {
     #[test]
     fn parse_code_block() {
         let input = "```rust\nfn main() {}\n```";
-        let doc = parse(input).expect("TODO: handle error");
+        let doc = parse(input).unwrap();
         assert_eq!(doc.blocks.len(), 1);
         if let Block::CodeBlock { language, content } = &doc.blocks[0] {
             assert_eq!(language.as_deref(), Some("rust"));
@@ -546,7 +547,7 @@ mod tests {
 
     #[test]
     fn parse_thematic_break() {
-        let doc = parse("---").expect("TODO: handle error");
+        let doc = parse("---").unwrap();
         assert_eq!(doc.blocks.len(), 1);
         assert_eq!(doc.blocks[0], Block::ThematicBreak);
     }