Parse AdGuard #$# CSS injection as style/remove actions

[ryanbr]

Summary

AdGuard's #$# CSS-injection syntax (e.g. businesshemden.com#$#.mnd-cookie-modal { display: none !important; }) was rejected outright as UnsupportedSyntax. This routes it through the same brace-body parser added in #678 for the uBO/ABP ##selector { ... } form, producing a Style (or remove: → Remove) action.

businesshemden.com#$#.mnd-cookie-modal { display: none !important; }
  →  Style(".mnd-cookie-modal" => "display: none !important;")

Details

• The between_sharps.starts_with('$') branch now consumes the $ and sets a flag instead of erroring. The existing ? and leading-@ handlers then cover the extended #$?# and exception #@$# forms with no extra code.
• A post-parse guard rejects any #$# rule that produced no action. This is the key safety check: ABP snippet injection also uses #$# but has no { ... } body (e.g. example.com#$#abort-on-property-read alert), and must stay unsupported rather than being misparsed as a plain CSS selector.
• #%# / #@%# (AdGuard JS/scriptlet) remain unsupported — scope is limited to the CSS style/remove form.

Behavior change

Two existing assertions in tests/unit/lists.rs that expected #$# / #@$# to be UnsupportedSyntax are updated to expect successful parsing — this is the intended contract change.

Tests

cargo fmt --check clean; full lib suite passes under both default and css-validation features. Adds adguard_css_injection (style, multi-class selector, remove:, ABP-snippet rejection, generic rejection) and adguard_css_injection_extended (#$?#).

[ryanbr]

Superseded by #686, opened from an internal branch so the Performance CI step can post its results comment. The failure here was a fork-PR token permission issue (pull_requests=write not granted to fork PRs), not a regression — all benchmark ratios were ≤ 1.00.