feat(authorization-obligations): group sibling handlers and find consistency anomalies

Task 2.2. Handlers on the same resource under the same module are siblings;
when at least min_siblings of them discharge an obligation and one does not,
that one is the anomaly and the discharging siblings are its evidence (Req 3.2).
A set with fewer members than the minimum is under-populated and never reported
(Req 3.3). A public route among authenticated siblings is a missing path guard
with the mapper's access classification as evidence (Req 3.4).

The sibling key is the module path without its suffix, so two same-named files
in different directories are never merged. Witness validity (an identity
constraint counts only when it binds the authenticated context) is one helper
shared by operations, siblings and the checker.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01T7XD7x5KonMdmGRkp63EQG

tests/test_obligation_operations.py

@@ -10,9 +10,12 @@ APP = (
     "from flask import request\n\n\n"
     "@app.route('/books/<title>')\n@login_required\ndef guarded(title):\n"
     "    return Book.query.filter_by(book_title=title).first()\n\n\n"
-    "@app.route('/books/mine/<title>')\ndef constrained(title):\n"
+    "@app.route('/books/mine/<title>')\n@login_required\ndef constrained(title):\n"
     "    user_id = request.user.id\n"
     "    return Book.query.filter_by(user_id=user_id, book_title=title).first()\n\n\n"
+    "@app.route('/books/shared/<title>')\ndef constrained_too(title):\n"
+    "    owner = current_user.id\n"
+    "    return Book.query.filter_by(owner=owner, book_title=title).first()\n\n\n"
     "@app.route('/books/any/<title>')\ndef leaky(title):\n"
     "    owner = request.args.get('owner')\n"
     "    return Book.query.filter_by(user_id=owner, book_title=title).first()\n\n\n"
@@ -39,9 +42,10 @@ def _entry(function: str, line: int, end: int, access: str, evidence: list[str])
 
 ENTRIES = [
     _entry("guarded", 6, 7, "authenticated", ["@login_required"]),
-    _entry("constrained", 11, 13, "public", []),
-    _entry("leaky", 17, 19, "public", []),
-    _entry("health", 23, 24, "public", []),
+    _entry("constrained", 12, 14, "authenticated", ["@login_required"]),
+    _entry("constrained_too", 18, 20, "public", []),
+    _entry("leaky", 24, 26, "public", []),
+    _entry("health", 30, 31, "public", []),
 ]
 
 
@@ -54,6 +58,7 @@ def test_find_operations_names_kind_fact_function_and_resource() -> None:
     assert [(op.function, op.kind, op.resource) for op in ops] == [
         ("guarded", "protected_read", "book"),
         ("constrained", "protected_read", "book"),
+        ("constrained_too", "protected_read", "book"),
         ("leaky", "protected_read", "book"),
     ]
     assert all(op.path == "app.py" and op.fact_id == "orm-read" and op.line > 0 for op in ops)
@@ -73,6 +78,7 @@ def test_find_discharges_reports_witness_kind_provenance_and_scope() -> None:
         by_function.setdefault(w.function, []).append((w.kind, w.provenance, w.scope))
     assert ("path_guard", "authenticated_context", "decorator") in by_function["guarded"]
     assert ("identity_constraint", "authenticated_context", "statement") in by_function["constrained"]
+    assert ("identity_constraint", "authenticated_context", "statement") in by_function["constrained_too"]
     assert ("identity_constraint", "request_input", "statement") in by_function["leaky"]  # present, but discharges nothing
     assert "health" not in by_function
     assert all(w.path == "app.py" and w.fact_id for w in witnesses)
@@ -87,6 +93,6 @@ def test_discharge_witness_covers_operation_only_when_kind_and_provenance_fit()
     ops = {op.function: op for op in find_operations(ir, facts, text=APP)}
     witnesses = find_discharges(ir, facts, load_facts().for_language("python"), text=APP, entries=ENTRIES)
     covering = {op.function: [w.kind for w in witnesses if covers(w, op)] for op in ops.values()}
-    assert covering["constrained"] == ["identity_constraint"]
+    assert covering["constrained"] == ["identity_constraint"] and covering["constrained_too"] == ["identity_constraint"]
     assert covering["leaky"] == []  # request-bound identity does not cover
     assert covering["guarded"] == []  # a path guard does not cover a protected read (only privileged actions)