feat(extract): add a Twig template extractor

[nitrobear]

Twig carries the entire presentation layer of Symfony, Drupal, Craft CMS and Grav projects, and none of it reached the graph: .twig was absent from CODE_EXTENSIONS, so the files were never walked in the first place. On a mid-sized Symfony app that is ~143 templates — the whole admin UI — invisible to query/explain/path.

The extractor follows the existing regex-based template extractors (blade.py, razor.py) rather than adding a grammar:

• inheritance and composition: {% extends %}, {% include %}, {% embed %}, {% import %}, {% from %}, {% use %}, plus the {{ include(...) }} function form
• {% block name %} definitions, scoped per file so that two templates defining content do not collapse onto one node
• path() / url() calls as references_route edges

That last relation is the one that pays: a route name emitted by a template resolves to the controller declaring it, so the presentation layer stops being a disconnected island in the graph.

Two details worth flagging for review:

• References resolve against the nearest ancestor directory named templates, which is the loader root convention in all four frameworks. When the target file exists, the node reuses that file's id, so {% extends "base.html.twig" %} becomes a real template-to-template edge instead of a dangling label. Namespaced (@AcmeBundle/...), out-of-tree and dynamic names stay as standalone label nodes.
• include() / path() / url() are only scanned inside {{ ... }} and {% ... %} regions. Templates carry a lot of inline <script>, and an unscoped scan happily matched a JS function literally named path(.

Covered by tests/test_twig_extraction.py (15 tests): dispatch, extension registration, each relation, whitespace-control markers ({%-), per-file block scoping, line numbers, the inline-script false positive above, unresolvable references, and unreadable files.