fix(taint): a filter returns its value argument; a source is the access, not a call around it

The apply_filters class from the second PMPro adjudication (2 of 3
remaining false positives): `apply_filters('pmpro_random_code', $code,
$this)` returned whatever $this carried, and a report's `$type` reached SQL
only as an extra filter argument. Two causes, both fixed generally:

- The dispatch fact now says what an apply call RETURNS: `returns` names
  the calls whose result is the dispatched value and `value_arg` its
  position (WordPress: apply_filters/apply_filters_ref_array return
  argument 2; do_action returns nothing). A path through such a call must
  ENTER by that argument -- the first of the run of the call's arguments
  before it, because the engine hops between sibling arguments
  ($_GET["c"] -> md5("x") -> apply_filters). Loaded and validated as data;
  the query knows no framework.
- A call was a source when the pattern appeared anywhere in its text, so
  `apply_filters(..., $_GET["c"])` and `sanitize_text_field($_POST[..])`
  were sources themselves -- the "a token matches the thing it names"
  invariant. A call is now a source when the pattern names its callee,
  receiver or the access it is; the inner access stays a source.

Controls: dispatch facts load/validate; a real-Joern case with request
data as filter context (quiet) and as the filtered value (found). 1,453
host and 18 query tests pass.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

src/openultrasast/ruleset/semantic/php.toml

@@ -208,6 +208,11 @@ parameterized = true
 id = "wordpress_hooks"
 register = ["add_action", "add_filter", "add_shortcode"]
 apply = ["apply_filters", "apply_filters_ref_array", "do_action", "do_action_ref_array"]
+# A filter returns its VALUE argument, filtered; its further arguments are context and never the result, and
+# an action returns nothing. Without this, `apply_filters('pmpro_random_code', $code, $this)` returned
+# whatever `$this` carried, and a random md5 fragment became an SQL injection.
+returns = ["apply_filters", "apply_filters_ref_array"]
+value_arg = 2
 
 # What a PHP repository's paths say about scope. `vendored` trees are out of the scan and out of the graph
 # (a bundled library is a separate unit); `tests` paths stay in scope but are not product, so their