Fix multi-hunk serialization bug, clippy warnings, and documentation issues

[lucazulian]

Bug fix:

• Fix Vec<Hunk>::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