feat(daily): milestones — the calm reward layer

Add the durable-states Milestone primitive and a milestone card to the
daily digest. A milestone marks a state REACHED, never days maintained:
a metric settled back inside its own range, a run of weeks in range, or
a fresh personal best. It is deliberately not a streak — the type carries
a reached-date and no running counter, so nothing here can be tuned into
a flame, and a lapse is invisible rather than mourned.

The primitive is a thin wrapper over engines that already exist: the pure
detectStreak band engine (return-to-range + sustained-in-range) and the
PersonalRecord detection output (new bests). Reached-once needs no new
storage — a milestone's reached-date is a stable fact, so it surfaces only
on the day it was reached and never resurfaces the next day.

The digest emits one calm success card per day when a milestone is freshly
reached, below an overdue dose and above ambient items, with a single
action into the metric's insight. Module- and data-gated: a disabled
insights account or a day with nothing reached shows nothing. Copy carries
no streak, loss, or failure vocabulary, in all six locales.

messages/en.json

@@ -8557,7 +8557,8 @@
       "viewCheckups": "View check-ups",
       "checkinKeep": "Keep it",
       "checkinAdjust": "Adjust",
-      "checkinLetGo": "Let it go"
+      "checkinLetGo": "Let it go",
+      "viewMilestone": "View"
     },
     "today": {
       "scoreLabel": "Health score",
@@ -8567,6 +8568,41 @@
       "readFullBriefing": "Read the full briefing",
       "worthALook": "Worth a look",
       "allClear": "Nothing needs your attention today."
+    },
+    "milestone": {
+      "metric": {
+        "restingHeartRate": "resting heart rate",
+        "heartRateVariability": "heart-rate variability",
+        "respiratoryRate": "respiratory rate",
+        "weight": "weight",
+        "generic": "this metric"
+      },
+      "record": {
+        "title": "A new {metric} best",
+        "body": "A personal best for your {metric} today — quietly noted, nothing to do."
+      },
+      "returnToBaseline": {
+        "title": "Back in range: {metric}",
+        "body": "Your {metric} has settled back inside your usual range. Whatever that was, it has passed."
+      },
+      "sustainedInRange": {
+        "week1": {
+          "title": "A week in range",
+          "body": "Your {metric} has held inside your usual range for a week now. Nice and steady."
+        },
+        "week2": {
+          "title": "Two weeks in range",
+          "body": "Your {metric} has stayed in your usual range for two weeks. Calm and steady."
+        },
+        "week3": {
+          "title": "Three weeks in range",
+          "body": "Three weeks of your {metric} sitting in your usual range. A real stretch of steadiness."
+        },
+        "week4": {
+          "title": "A month in range",
+          "body": "A full month with your {metric} in your usual range. Beautifully steady."
+        }
+      }
     }
   }
 }