P20.1: FPS counter — env-gated dev overlay (M3TE_FPS), off by default

Add a small top-left FPS readout for gauging frame cost while tuning the
organic animations. Gated behind the M3TE_FPS startup env pin (read like the
other M3TE_* hooks); unset/=0 renders nothing, so default play and every
committed golden stay byte-identical.

- main.sx: g_fps_on (from M3TE_FPS) + g_fps_avg_dt, an EMA of delta_time
  (FPS_DT_SMOOTH=0.9) advanced only on the gated path; build_ui passes the
  smoothed FPS + flag into BoardView. delta_time is real wall-clock even when
  M3TE_ANIM_TIME pins the scene, so the counter stays live while frozen.
- board_view.sx: BoardView.fps_on/fps fields + render_fps_overlay — "FPS n"
  in the top-left safe-area corner (clear of notch/Dynamic Island + the HUD),
  dark grape text over a bright halo. Drawn last, only when fps_on.
- README.md: document M3TE_FPS (sim SIMCTL_CHILD_ + device devicectl env).
- goldens/p20_fps.png: FPS overlay over the resting board (M3TE_FPS=1,
  M3TE_ANIM_TIME=0); FPS digits are dynamic, rest pinned == p6_idle_t0 region.

Verified: ios-sim build + 22 logic tests green. Unset capture's board+HUD
region is byte-identical to goldens/p6_idle_t0.png; the only ON-vs-OFF delta
is the top-left FPS text box.

board_view.sx

@@ -60,6 +60,15 @@ HUD_PANEL    :: Color.{ r = 92,  g = 46,  b = 150, a = 224 }; // bright grape ca
 HUD_PANEL_HI :: Color.{ r = 196, g = 138, b = 240, a = 92  }; // glossy top sheen
 HUD_PANEL_RIM:: Color.{ r = 236, g = 204, b = 255, a = 150 }; // bright candy rim
 
+// FPS dev overlay (P20.1): a small corner readout, OFF unless M3TE_FPS pins it on
+// (so default play + every golden are unchanged). Pinned to the top-left of the
+// safe area — clear of the centered notch / Dynamic Island and the centered HUD.
+// Dark grape text over a bright halo keeps it legible on the light lavender art.
+FPS_FONT    :f32: 22.0;
+FPS_PAD     :f32: 8.0;
+FPS_TEXT   :: Color.{ r = 40,  g = 16,  b = 64,  a = 235 }; // dark grape, readable on lavender
+FPS_TEXT_SH:: Color.{ r = 255, g = 255, b = 255, a = 170 }; // bright halo for contrast
+
 // Win/lose banner (P12.2): a warm dim over the board, a glossy candy panel, the
 // win/lose headline, and a playful restart button. Built from text + rects only —
 // the engine's image path can't tint/fade at draw time (issue 0002), but rects and
@@ -229,6 +238,11 @@ BoardView :: struct {
     // Seed for `restart`: the same fixed seed main seeded the board with, so the
     // restart button reproduces the identical starting level.
     seed: s64;
+    // FPS dev overlay (P20.1). `fps_on` gates the corner readout (off by default,
+    // set only by the M3TE_FPS env pin); `fps` is the smoothed reciprocal frame
+    // rate computed in the frame loop. Purely a render overlay.
+    fps_on: bool;
+    fps: f32;
 
     // Where the grid sits + the touch↔cell mapping. Recomputed each render /
     // event from the current frame so the hit-test matches what was drawn.
@@ -601,6 +615,21 @@ BoardView :: struct {
         ctx.add_text(bfr, btxt, bfont, BANNER_BTN_TEXT);
     }
 
+    // FPS dev overlay (P20.1): a small "FPS n" readout pinned to the top-left of
+    // the safe area, on top of everything. Drawn only when fps_on (the M3TE_FPS
+    // pin) is set, so the unset render path is byte-identical. A bright halo under
+    // the dark text keeps the digits legible over the light background art.
+    render_fps_overlay :: (self: *BoardView, ctx: *RenderContext, frame: Frame) {
+        n := cast(s64) (self.fps + 0.5);
+        txt := format("FPS {}", n);
+        sz := measure_text(txt, FPS_FONT);
+        x := frame.origin.x + self.safe.left + FPS_PAD;
+        y := frame.origin.y + self.safe.top + FPS_PAD;
+        f := Frame.make(x, y, sz.width, sz.height);
+        ctx.add_text(Frame.make(f.origin.x + 1.0, f.origin.y + 1.5, f.size.width, f.size.height), txt, FPS_FONT, FPS_TEXT_SH);
+        ctx.add_text(f, txt, FPS_FONT, FPS_TEXT);
+    }
+
     // Restart action behind the banner's button: reseed the SAME starting level
     // through the model (board.restart) and drop every transient view layer
     // (selection, in-flight drag, move animation, FX, and the per-gem landing
@@ -694,6 +723,12 @@ impl View for BoardView {
         if self.banner_up() {
             self.render_banner(ctx, level_status(self.board));
         }
+
+        // 7. FPS dev overlay (P20.1), on top of everything. Off by default; only
+        //    renders when M3TE_FPS pinned it on, so the unset path is unchanged.
+        if self.fps_on {
+            self.render_fps_overlay(ctx, frame);
+        }
     }
 
     // Touch input. A press records the drag start; the release resolves the