…issues

Bug fix:
- Fix Vec<Hunk<T>>::to_patch() concatenating hunks without newline separator,
  producing invalid unified diff format (e.g. ' d@@ -6,4 +6,4 @@')

Clippy fixes:
- Replace if/else if chain with match + cmp in apply() (comparison_chain)
- Replace redundant closures with method references (redundant_closure_for_method_calls)
- Add missing semicolons in traceback() (semicolon_if_nothing_returned)
- Remove unnecessary semicolon in HunkBuilder::process()
- Replace wildcard import with explicit imports in diffable.rs

Documentation fixes:
- Fix broken rustdoc links: Hunk -> patch::Hunk, Change -> recursive::Change
- Add backticks around Edit::Equal in Hunk doc comment (doc_markdown)
- Add backticks around PatchError in apply() doc comment
- Add # Errors section to apply() and FromPatch::from_patch() docs

Tests:
- Add test_multi_hunk_patch_format to verify @@ headers are on their own line
- Add test_multi_hunk_roundtrip to verify serialize/deserialize with multiple hunks