refactor(routes): wire DELETE on path-keyed workstreams/{ws_id}/send

Pre-flight for the legacy URL adapter removal: the path-keyed
`/v1/api/workstreams/{ws_id}/send` route only mounted POST today;
the dequeue handler was reachable only via the legacy
`DELETE /v1/api/send` body-keyed URL through `_make_method_dispatch`.

Add a new `dequeue: Handler | None = None` slot on
`SharedSessionVerbHandlers` next to `send`, mounted as a second
`Route` on the same path with `methods=["DELETE"]` (two distinct
Routes rather than collapsing methods on one Route — different
handler callables, and collapsing would force the same
method-dispatch wrapper this cleanup is tearing out).

Wire `dequeue=dequeue_handler` in `turnstone/server.py`'s
`SharedSessionVerbHandlers(...)` call so DELETE on the path-keyed
shape works in the same merge as the legacy mount removal.

Adds a regression-locking test covering both the POST+DELETE and
the dequeue-alone cases.

tests/test_session_routes.py

@@ -119,6 +119,34 @@ def test_attachment_routes_mount_when_quartet_provided() -> None:
     ) in paths
 
 
+def test_send_mounts_post_and_delete_when_dequeue_provided() -> None:
+    """``handlers.send`` mounts POST {prefix}/{ws_id}/send and
+    ``handlers.dequeue`` mounts DELETE on the same path. The two
+    routes register as separate ``Route`` entries with disjoint
+    method sets — Starlette dispatches by (path, method)."""
+    routes: list[Any] = []
+    register_session_routes(
+        routes,
+        prefix="/api/workstreams",
+        handlers=SharedSessionVerbHandlers(send=_stub, dequeue=_stub),
+    )
+    paths = {(p, m) for p, m in _route_paths(routes)}
+    assert ("/api/workstreams/{ws_id}/send", frozenset({"POST"})) in paths
+    assert ("/api/workstreams/{ws_id}/send", frozenset({"DELETE"})) in paths
+
+    # ``dequeue`` is independent of ``send`` — providing it alone
+    # mounts only the DELETE half (no POST regression).
+    routes_dequeue_only: list[Any] = []
+    register_session_routes(
+        routes_dequeue_only,
+        prefix="/api/workstreams",
+        handlers=SharedSessionVerbHandlers(dequeue=_stub),
+    )
+    paths_dequeue_only = {(p, m) for p, m in _route_paths(routes_dequeue_only)}
+    assert ("/api/workstreams/{ws_id}/send", frozenset({"DELETE"})) in paths_dequeue_only
+    assert ("/api/workstreams/{ws_id}/send", frozenset({"POST"})) not in paths_dequeue_only
+
+
 def test_close_legacy_mounts_when_handler_provided() -> None:
     """The legacy body-keyed close (``POST {prefix}/close``) mounts
     when ``handlers.close_legacy`` is non-``None`` — there is no

turnstone/core/session_routes.py

@@ -482,6 +482,7 @@ class SharedSessionVerbHandlers:
     # Per-``{ws_id}`` interaction (coord shape today; interactive
     # adopts these in Priority 1's worker dispatch unification)
     send: Handler | None = None  # POST {prefix}/{ws_id}/send
+    dequeue: Handler | None = None  # DELETE {prefix}/{ws_id}/send
     approve: Handler | None = None  # POST {prefix}/{ws_id}/approve
     plan: Handler | None = None  # POST {prefix}/{ws_id}/plan
     cancel: Handler | None = None  # POST {prefix}/{ws_id}/cancel
@@ -566,6 +567,8 @@ def register_session_routes(
         routes.append(Route(f"{p}/{{ws_id}}/title", handlers.set_title, methods=["POST"]))
     if handlers.send is not None:
         routes.append(Route(f"{p}/{{ws_id}}/send", handlers.send, methods=["POST"]))
+    if handlers.dequeue is not None:
+        routes.append(Route(f"{p}/{{ws_id}}/send", handlers.dequeue, methods=["DELETE"]))
     if handlers.approve is not None:
         routes.append(Route(f"{p}/{{ws_id}}/approve", handlers.approve, methods=["POST"]))
     if handlers.plan is not None:

turnstone/server.py

@@ -3556,6 +3556,7 @@ def create_app(
             refresh_title=refresh_workstream_title,
             set_title=set_workstream_title,
             send=send_handler,  # lifted: shared body (P1.5)
+            dequeue=dequeue_handler,  # lifted (P1.5) — DELETE /send
             approve=approve_handler,  # lifted: shared body
             cancel=cancel_handler,  # lifted: shared body
             events=events_handler,  # lifted: shared body