feat: per-class max box fraction (REQ-188)

- labeling/preview/autolabel: max_box_frac + per-class max_box_fracs
  ceiling filter (0=none, 1=off) before NMS, mirrors min_box_frac
- exemplar review-assist: ceiling before max_detections truncation;
  ExemplarLabelRequest + filter panel 'Max box size' slider
- ClassParamsTable: MaxBox column; copy line now REQ-186 amended
  order: class <name> conf <v> iou <v> minbox <v> maxbox <v> container
- docs: design/ui-spec/tasks updated (incl. stale 4-slider narrative)

docs/ui-spec.md

@@ -450,7 +450,7 @@ Two columns inside one dialog (920 px wide, max 96 vw / 90 vh, scrollable).
 - NMS IoU threshold — 0…0.9 step 0.05, default 0.0.
 - Min box size (fraction of frame) — 0…0.5 step 0.005, default 0, shown as a percentage.
 - **Per-class overrides** (REQ-181) — a compact table below the sliders, one row per currently
-  selected class: `Conf` / `IoU` / `MinBox` number inputs and a **Container** checkbox
+  selected class: `Conf` / `IoU` / `MinBox` / `MaxBox` number inputs and a **Container** checkbox
   (REQ-184). Each input's placeholder shows the current global value and an empty input
   inherits it; only filled cells are sent, as
   `class_params` on **both** the preview and the job request, so preview and run cannot
@@ -465,7 +465,7 @@ Two columns inside one dialog (920 px wide, max 96 vw / 90 vh, scrollable).
 - **Copy (REQ-186)** — small right-aligned ghost button above the overrides table; label
   `Copy`, flips to `Copied` for 1.5 s inside an `aria-live` span, and on failure silently
   stays `Copy`. One line per selected class, e.g.
-  `class sack conf 0.4 iou 0.6 minbox 0.005 container false` — an empty input copies the
+  `class sack conf 0.4 iou 0.6 minbox 0.005 maxbox 1 container false` — an empty input copies the
   global value. Present in the engine-chooser modal **and** the mass modal (shared
   component).
 - `ClassPromptPanel`:
@@ -1024,13 +1024,14 @@ over the frame** — the shapes it governs are drawn on the canvas, and a card o
 covers the thing being judged. It scrolls itself into view on mount, because the sidebar
 scrolls and a decision the user cannot see is not a decision.
 
-Its four sliders, re-previewing on a 250 ms debounce:
+Its five sliders, re-previewing on a 250 ms debounce:
 
 | Key | Label | Range | Default | Hint |
 |---|---|---|---|---|
 | `threshold` | Confidence | 0.05–0.95 | 0.5 | lower finds more, and more junk |
 | `iou_threshold` | Overlap (NMS) | 0.1–1 | 0.8 | boxes overlapping this much are one object; lower deletes more |
 | `min_box_frac` | Min box size | 0–0.05 | 0.002 | shown as % of frame; higher deletes more |
+| `max_box_frac` | Max box size | 0–1 | 1.0 | shown as % of frame; boxes bigger than this are junk, 100% = off |
 | `max_detections` | Max shapes | 5–300 | 100 | keep only the highest-scoring N |
 
 Panel header: `Auto-label "<class>"` + *Undo* (drops the last example drawn). Status line:
@@ -1062,7 +1063,7 @@ POST /api/frames/{id}/exemplar-label
 { "exemplars": [{ "box": [x0,y0,x1,y1], "positive": true }],
   "class_id": 0,
   "threshold": 0.5, "iou_threshold": 0.8,
-  "min_box_frac": 0.002, "max_detections": 100,
+  "min_box_frac": 0.002, "max_box_frac": 1.0, "max_detections": 100,
   "apply": false }
 → { "shapes": [{ "geometry", "score", "source": "manual"|"auto" }],
     "applied": false, "redetected": true, "message": null,
@@ -1072,7 +1073,7 @@ POST /api/frames/{id}/exemplar-label
 Server-side, positives become `source="manual"` and survive a batch re-run; what SAM3 adds
 becomes `source="auto"` and is replaceable. Detections overlapping a negative by ≥0.3 are
 dropped; ones overlapping a positive by ≥0.6 are dropped as already covered. The panel's
-filters run first, in the batch job's order: area floor → NMS → cap.
+filters run first, in the batch job's order: area floor → ceiling → NMS → cap.
 
 **GPU-lock timeout is 5 s here** (versus 20 s for `assist`) because this fires from a mouse
 gesture. On timeout the response carries `redetected: false`, a message the panel shows, and
@@ -1527,8 +1528,8 @@ Shortcuts).
 - Dua mode: **Draw** / **Select** (`V`). Select mode tidak pernah mengubah geometri; Draw mode
   tidak marquee.
 - **Di draw mode, drag bukan menggambar kotak — itu adalah *contoh*.** User menggambar satu
-  instance, SAM3 mencari sisanya, hasilnya di-*preview*, di-*tune* lewat 4 slider (Confidence,
-  Overlap/NMS, Min box size, Max shapes), lalu baru **Apply** yang menulis. Shift-drag = contoh
+  instance, SAM3 mencari sisanya, hasilnya di-*preview*, di-*tune* lewat 5 slider (Confidence,
+  Overlap/NMS, Min box size, Max box size, Max shapes), lalu baru **Apply** yang menulis. Shift-drag = contoh
   negatif ("bukan ini"). Panel filter duduk di **sidebar**, bukan menutupi frame.
 - Tahan `S` = SAM3 click-assist (satu box → satu shape).
 - Keyboard-first: `V ← → A X U N C T 1–9 Del H Esc Enter`. Semua aksi punya shortcut; panel