add session browsing, calibration diagnostics and the 2D layout preset

Phase 2 of the review: browse the whole session instead of only the rolling
window, diagnose a calibration from the recorded points, and provide the
reviewed two-panel layout.

What
- SessionStore gains extent(), visible(t0, t1, max_points) and
  statistics(t0, t1, unit). visible() walks only the blocks that overlap the
  window and decimates by taking each bucket's minimum and maximum, so a spike
  survives decimation and browsing a long session does not copy all of it.
- ForcePlotsPanel gains follow/browse modes, a whole-session overview strip
  with the current window, pan and wheel zoom, a rubber-band zoom toggle,
  a shared time cursor across all three channels, and a selection readout
  (duration, valid samples, per-axis peak/mean/std).
- CalibrationPoint records a bounded per-axis trace, an excluded flag, a
  validation flag and its fit residual. CalibrationRecorder gains
  set_excluded/set_validation/fit; the fit is a least-squares line over the
  non-excluded, non-validation points with per-point residuals and an
  independent validation error.
- The calibration table becomes the reviewed diagnostic view
  (reference load | FN | FX | FY | stability | state) with the residual shown
  next to the loaded axis, plus exclude/restore and mark-validation actions and
  a fit summary line. Double clicking a row opens the point's three-axis trace.
- Add a layout preset: free layout or 2D mode (depth map on the left, force
  charts on the right, calibration folded away), persisted in preferences.

Why
- Browsing is a display pause, not a capture pause: the record channel keeps
  running while the charts show an older window.
- Excluding a point removes it from the fit and from the exported calibration
  but keeps it on record, so the evidence for the decision is not destroyed.
- The non-loaded axes are recorded but never given a reference load: their true
  load is unknown during a single-axis calibration, so it stays empty.

Verification
- shared/tests/test_record_channel.py grows to 18 cases, covering extent,
  peak-preserving decimation, windowed reads, inverted/empty windows, unit-aware
  statistics and the time origin.
- tools/ui_check.py gains "session browsing" and "layout preset" sections
  (223 checks, 0 failures overall).
- apps/studio/tests/test_ui_smoke.py covers the same invariants.

Not in this change: synchronised playback of video, depth and curves. It needs
a decision on how depth is stored for replay (per-frame PNG, reduced-rate PNG,
or offline recomputation); the cursor plumbing (cursor_moved / set_cursor) is
already in place for it.

apps/studio/config/locales/en_US.json

@@ -102,7 +102,24 @@
     "fn": "FN",
     "fx": "FX",
     "fy": "FY",
-    "no_data": "—"
+    "no_data": "—",
+    "mode_follow": "Following live",
+    "mode_browse": "Browsing history (display paused; capture and recording continue)",
+    "return_latest": "Back to latest",
+    "return_latest_tip": "Leave history browsing and follow the newest data again",
+    "overview": "Whole session",
+    "overview_tip": "Fit the entire session; drag the overview strip to move the window",
+    "box_zoom": "Box zoom",
+    "box_zoom_tip": "Switch to rubber-band zoom (drag a region on the chart)",
+    "selection": "Selection {duration}s / {samples} samples — {detail}",
+    "selection_empty": "No valid samples in the selection",
+    "browse_only_tip": "Drag or scroll on the chart to browse history"
+  },
+  "layout": {
+    "default": "Free layout",
+    "preset_2d_name": "2D mode (depth + charts)",
+    "preset_2d": "2D mode: depth map on the left, force charts on the right, one shared time axis",
+    "preset_default": "Free layout restored"
   },
   "record": {
     "label": "Record",
@@ -135,10 +152,40 @@
     "sn_placeholder": "Serial number (optional)",
     "live_raw_label": "Live raw",
     "live_raw": "Live raw  FN={fn}  FX={fx}  FY={fy}",
-    "col_weight": "Weight (N)",
+    "col_weight": "Reference load",
+    "col_fn": "FN mean",
+    "col_fx": "FX mean",
+    "col_fy": "FY mean",
+    "col_stability": "Stability",
+    "col_state": "State",
     "col_raw": "Raw mean",
     "col_n": "n",
     "col_std": "σ",
+    "state_ok": "OK",
+    "state_excluded": "Excluded",
+    "state_validation": "Validation",
+    "state_exclude": "Exclude",
+    "state_exclude_tip": "Drop the selected point from the fit and the export (kept on record, reversible)",
+    "state_restore": "Restore",
+    "mark_validation": "Mark validation",
+    "mark_validation_tip": "Hold the selected point out of the fit and use it to test the fit",
+    "excluded": "Point excluded from the fit",
+    "restored": "Point restored",
+    "marked_validation": "Point marked as a validation point",
+    "unmarked_validation": "Validation mark removed",
+    "fit_insufficient": "A fit needs at least 2 usable points (currently {count})",
+    "fit_summary": "Fit {channel}: N = {gain} × raw + {offset} ({count} points, max residual {residual} N)",
+    "fit_validation": "Independent validation on {count} point(s), max deviation {error} N",
+    "point_title": "Calibration point {channel} @ {load} N",
+    "point_channel": "Loaded axis",
+    "point_direction": "Direction",
+    "point_round": "Round",
+    "point_duration": "Actual sampling time",
+    "point_duration_value": "{actual} s (requested {requested} s)",
+    "point_samples": "Samples",
+    "point_samples_value": "{valid} valid / {total} total",
+    "point_axes": "Three-axis mean ± std",
+    "point_residual": "Fit residual",
     "col_actions": "",
     "recording": "Recording calibration samples…",
     "recording_stop_tip": "Click to cancel this calibration recording (always available)",