[SizzleUnrlsd]

Closes #183.

A project tag library registering a filter of a built-in's name replaces the built-in in every template loading it before applying the filter, as Django's parser does. {% load custom %}{{ bio|striptags }} was reported as a reachable, and with attacker input an exploitable, call to django.template.defaultfilters.striptags even when app/templatetags/custom.py registers its own striptags: the project's function is ordinary code, analysed as any other, and the template's call is no evidence about the built-in.

• _read_scopes resolves each filter where it is applied: the built-in, unless the latest applicable {% load %} so far names a project library that certainly registers the name (nothing recorded) or may (the load is kept as unresolved). Loads are ordered as Django parses them, {% load a from lib %} selects, Django's own libraries change nothing.
• Libraries are the templatetags/*.py modules discover_files already lists, read with the HIR and the semantic layers: register must resolve to django.template.Library whatever the import spelling; the decorator and call forms of register.filter are read, at module level or under if, try or a function; tag registrars are ignored. Anything the engine cannot follow, a name not written as a constant, unpacked arguments, register passed elsewhere (to a call or a decorator), filters= or register.filters, another Library attribute, a file it cannot read or parse, makes the whole library uncertain, fail-closed.
• Uncertainty flows through the existing unread channel, app/templates/app/profile.html:1 loads 'custom', not found | not read | found in several files, so Coverage.unread_templates and the VEX under_investigation status need no change. OPTIONS["libraries"] and OPTIONS["builtins"] are not read, and a library found under no templatetags directory, such as one an installed package ships, is uncertain too (documented).
• escaped_templates is untouched: a template loading an overriding library is still not established to escape.

Tests: tests/test_template_filter_overrides.py (60 cases, 52 failing before the change: reproduction, 17 registration forms, 7 import spellings, load order, selective loads, 11 unreadable forms, two apps agreeing or not, an override calling strip_tags reported at the library's line only, calls/filters consequences); pytest -m "not regression" 1483 passed, 2 skipped; regression corpus 19 passed, snapshots unchanged; ruff check . and mypy clean.