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/cpg/backend.py

@@ -181,7 +181,7 @@ FRONTEND_BUILD_ATTEMPTS = 4
 # across 2,500 requests made a 42.5MB request file of which 41.7MB was the same string over and over. Joern
 # parsed that with ujson inside a 2GB heap and the whole batch died in ForkJoinPool, which the driver
 # correctly reported as `query_failed` for all 2,500 regions -- a whole-repository scan that decided nothing.
-_SHARED_REQUEST_FIELDS = ("hookCallbacks", "dispatchApply", "contextFilter", "contextPaths")
+_SHARED_REQUEST_FIELDS = ("hookCallbacks", "dispatchApply", "dispatchValue", "contextFilter", "contextPaths")
 
 Runner = Callable[..., subprocess.CompletedProcess[str]]