docs: implementation plan for SAM21 segmentation in grasp pipeline

7 task TDD-first: pytest scaffold, depth_align (vectorised IR→RGB),
Sam21Client (httpx mock unit tests), _deps + config wiring, refactor
estraendo _points_via_bbox, branch SAM21 con fallback, verifica live.
Allinea config key a base_url (consistente con altri servizi).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/superpowers/specs/2026-05-11-sam21-segmentation-design.md

@@ -135,7 +135,7 @@ def sam21() -> Sam21Client:
     cfg = get_config()
     s = cfg.get("services", {}).get("sam21", {})
     return Sam21Client(
-        base_url=s.get("url", "http://localhost:8002"),
+        base_url=s.get("base_url", "http://localhost:8002"),
         timeout_s=float(s.get("timeout_s", 10.0)),
     )
 ```
@@ -222,7 +222,7 @@ failure of the grasp request.
 ```yaml
 services:
   sam21:
-    url: http://localhost:8002
+    base_url: http://localhost:8002
     timeout_s: 10.0
 
 grasp: