fix(measurements): wire walking step length and walking speed

The iOS team had gated both client-side waiting on the server map.
Both flow raw SI on the wire (metres and metres per second
respectively) — the server-side ×100 scaling the percent gait
metrics use does NOT apply here. The convention block at the top of
apple-health-mapping.ts now spells the split out so a future
gait-metric addition picks the right path.

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

docs/api/openapi.yaml

@@ -176,6 +176,8 @@ paths:
               - WALKING_HEART_RATE_AVERAGE
               - WALKING_ASYMMETRY
               - WALKING_DOUBLE_SUPPORT
+              - WALKING_STEP_LENGTH
+              - WALKING_SPEED
         - in: query
           name: from
           schema:
@@ -307,6 +309,8 @@ paths:
                     - WALKING_HEART_RATE_AVERAGE
                     - WALKING_ASYMMETRY
                     - WALKING_DOUBLE_SUPPORT
+                    - WALKING_STEP_LENGTH
+                    - WALKING_SPEED
                 value:
                   type: number
                 measuredAt:
@@ -2343,6 +2347,8 @@ components:
             - WALKING_HEART_RATE_AVERAGE
             - WALKING_ASYMMETRY
             - WALKING_DOUBLE_SUPPORT
+            - WALKING_STEP_LENGTH
+            - WALKING_SPEED
         value:
           type: number
         unit: