Trace: anchor events to kernel boot, mark first GET /

- Trace reads /proc/stat btime and /proc/uptime at import so every
  event in /api/diag/timing can be expressed as 'seconds since
  power-on' (uptime_at_anchor + ev.t).
- Web.StaticFileHandler.prepare emits 'web.first_root_get' the first
  time chromium hits / or /index.html, so we can see when the kiosk
  browser actually started loading the UI on cold boot.

src/py/bbctrl/Trace.py

@@ -37,6 +37,38 @@ _events = []        # list of dicts: {t, name, fields}
 _ui_timing = None   # last timeline POSTed by the browser
 
 
+def _read_kernel_anchors():
+    """Return (btime_wall, uptime_at_anchor) so we can express bbctrl events
+    in seconds since kernel boot.
+
+    btime_wall: wall-clock epoch seconds when the kernel booted (from
+    /proc/stat 'btime').
+    uptime_at_anchor: monotonic offset (seconds since kernel boot) at the
+    moment Trace was imported. Equivalent to (Trace anchor) - btime
+    in wall time, but read directly from /proc/uptime so it isn't
+    sensitive to wall-clock skew.
+    """
+    btime = None
+    uptime_at_anchor = None
+    try:
+        with open('/proc/stat') as f:
+            for line in f:
+                if line.startswith('btime '):
+                    btime = int(line.split()[1])
+                    break
+    except Exception:
+        pass
+    try:
+        with open('/proc/uptime') as f:
+            uptime_at_anchor = float(f.read().split()[0])
+    except Exception:
+        pass
+    return btime, uptime_at_anchor
+
+
+_btime_wall, _uptime_at_anchor = _read_kernel_anchors()
+
+
 def now():
     return time.monotonic() - _t0_monotonic
 
@@ -91,6 +123,14 @@ def set_ui_timing(data):
     _ui_timing = data
 
 
+def _current_uptime():
+    try:
+        with open('/proc/uptime') as f:
+            return float(f.read().split()[0])
+    except Exception:
+        return None
+
+
 def timeline():
     with _lock:
         events = list(_events)
@@ -102,6 +142,11 @@ def timeline():
         'pid': os.getpid(),
         'events': events,
         'ui': _ui_timing,
+        # Kernel-boot anchors so the timeline can be expressed in
+        # "seconds since power on".
+        'btime_wall': _btime_wall,
+        'uptime_at_anchor': _uptime_at_anchor,
+        'uptime_now': _current_uptime(),
     }