The scanning for "interesting lines" (function and class definition headers shown on collapsed diff chunks) lived in the `Differ`, driven by regex tables in diffviewer/filetypes.py. This change moves it into a new `diffviewer.interesting_lines` module, in preparation for detecting these lines with tree-sitter. The new module hosts the regex tables, an `InterestingLine` type, and a `get_interesting_lines()` entry point (currently regex-only). `DiffChunkGenerator.generate_chunks()` now computes the interesting lines for both file revisions directly, and `_get_interesting_headers()` reads those instead of asking the differ. The legacy Differ API keeps working for third parties: - `Differ.add_interesting_line_regex()` is deprecated for removal in Review Board 11.0 but still contributes results for now. - `Differ.get_interesting_lines()` runs the regex scanner, since no parsed tree is available at that layer. `DiffChunkGenerator` is the supported path going forward. - `filetypes.py` re-exports `HEADER_REGEXES` and `HEADER_REGEX_ALIASES` for backwards compatibility. The interesting-lines tests are rewritten as parametrized pytest cases with no-op database fixtures. Testing Done: Ran unit tests.