add opt-in render profiler instrumentation

Gated behind -Djprototerm.profile=true (or JPROTOTERM_PROFILE=1), accumulates
per-frame nanos into snapshot/fingerprint/draw/frame-total buckets and dumps
to stderr every N renders. Splits the three suspected render costs: native
snapshot marshaling, fingerprint hashing, and canvas draw recording.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/java/com/gregor/jprototerm/RenderProfiler.java

@@ -0,0 +1,76 @@
+package com.gregor.jprototerm;
+
+/**
+ * Lightweight render profiler, disabled unless {@code -Djprototerm.profile=true} (or the
+ * {@code JPROTOTERM_PROFILE=1} environment variable) is set. It accumulates wall-clock nanos
+ * into a handful of buckets and prints aggregate per-frame stats to stderr every
+ * {@code jprototerm.profile.frames} render invocations (default 120).
+ *
+ * <p>All render work runs on the JavaFX application thread, so the accumulators are plain
+ * fields with no synchronization.
+ *
+ * <p>Caveat: JavaFX canvas drawing is deferred to the QuantumRenderer thread, so the
+ * {@link #DRAW} bucket measures only the cost of <em>recording</em> draw commands, not the
+ * GPU paint. Pair this with {@code -Djavafx.pulseLogger=true} to see the render-thread side.
+ */
+final class RenderProfiler {
+    static final int SNAPSHOT = 0;
+    static final int FINGERPRINT = 1;
+    static final int DRAW = 2;
+    static final int FRAME = 3;
+    private static final int BUCKETS = 4;
+    private static final String[] NAMES = {"snapshot", "fingerprint", "draw", "frame-total"};
+
+    private static final boolean ENABLED =
+            Boolean.getBoolean("jprototerm.profile") || "1".equals(System.getenv("JPROTOTERM_PROFILE"));
+    private static final int DUMP_FRAMES = Integer.getInteger("jprototerm.profile.frames", 120);
+
+    private static final long[] totalNanos = new long[BUCKETS];
+    private static final long[] counts = new long[BUCKETS];
+    private static int frames;
+
+    private RenderProfiler() {
+    }
+
+    static boolean enabled() {
+        return ENABLED;
+    }
+
+    /** Returns a start timestamp, or 0 when profiling is disabled. */
+    static long start() {
+        return ENABLED ? System.nanoTime() : 0L;
+    }
+
+    /** Records the time elapsed since {@code startNanos} into {@code bucket}. */
+    static void stop(int bucket, long startNanos) {
+        if (!ENABLED) {
+            return;
+        }
+        totalNanos[bucket] += System.nanoTime() - startNanos;
+        counts[bucket]++;
+    }
+
+    /** Marks the end of one render invocation; dumps and resets every {@code DUMP_FRAMES}. */
+    static void frame() {
+        if (!ENABLED) {
+            return;
+        }
+        if (++frames < DUMP_FRAMES) {
+            return;
+        }
+        dump();
+    }
+
+    private static void dump() {
+        StringBuilder sb = new StringBuilder(192);
+        sb.append("[render-profile] ").append(frames).append(" renders");
+        for (int i = 0; i < BUCKETS; i++) {
+            double totalMs = totalNanos[i] / 1_000_000.0;
+            sb.append(String.format(" | %s %.3fms/f (n=%d)", NAMES[i], totalMs / frames, counts[i]));
+            totalNanos[i] = 0;
+            counts[i] = 0;
+        }
+        System.err.println(sb);
+        frames = 0;
+    }
+}

src/main/java/com/gregor/jprototerm/TerminalPaneNode.java

@@ -91,7 +91,9 @@ final class TerminalPaneNode extends Region {
 
     void renderFull(boolean active) {
         prepareGeometry();
+        long snapshotStart = RenderProfiler.start();
         RenderStateSnapshot snapshot = pane.snapshotFull();
+        RenderProfiler.stop(RenderProfiler.SNAPSHOT, snapshotStart);
         long renderedVersion = pane.snapshotVersion();
         boolean withKitty = pane.kittyEnabled() && hasKittyGraphics();
         updateRowsFull(snapshot);
@@ -102,6 +104,13 @@ final class TerminalPaneNode extends Region {
     }
 
     void renderIncremental(boolean active) {
+        long frameStart = RenderProfiler.start();
+        renderIncrementalBody(active);
+        RenderProfiler.stop(RenderProfiler.FRAME, frameStart);
+        RenderProfiler.frame();
+    }
+
+    private void renderIncrementalBody(boolean active) {
         boolean geometryChanged = prepareGeometry();
         boolean withKitty = pane.kittyEnabled() && hasKittyGraphics();
         if (drawnContentVersion == Long.MIN_VALUE || geometryChanged || withKitty) {
@@ -113,7 +122,9 @@ final class TerminalPaneNode extends Region {
             return;
         }
 
+        long snapshotStart = RenderProfiler.start();
         RenderStateSnapshot snapshot = pane.snapshot();
+        RenderProfiler.stop(RenderProfiler.SNAPSHOT, snapshotStart);
         long renderedVersion = pane.snapshotVersion();
         int dirty = snapshot == null ? DIRTY_FULL : snapshot.dirty();
         if (dirty == DIRTY_FULL) {
@@ -167,7 +178,9 @@ final class TerminalPaneNode extends Region {
         Set<Integer> liveRows = new HashSet<>();
         for (RenderRow row : snapshot.renderRows()) {
             TerminalRowNode node = rowNode(row.row());
+            long fpStart = RenderProfiler.start();
             long fingerprint = rowFingerprint(row);
+            RenderProfiler.stop(RenderProfiler.FINGERPRINT, fpStart);
             node.render(row);
             rowFingerprints.put(row.row(), fingerprint);
             liveRows.add(row.row());
@@ -197,7 +210,9 @@ final class TerminalPaneNode extends Region {
                 continue;
             }
             TerminalRowNode node = rowNode(row.row());
+            long fpStart = RenderProfiler.start();
             long fingerprint = rowFingerprint(row);
+            RenderProfiler.stop(RenderProfiler.FINGERPRINT, fpStart);
             node.renderChanged(row);
             rowFingerprints.put(row.row(), fingerprint);
         }
@@ -212,7 +227,9 @@ final class TerminalPaneNode extends Region {
             return Set.of();
         }
 
+        long shiftStart = RenderProfiler.start();
         ShiftPlan plan = detectShift(snapshot, changedRows);
+        RenderProfiler.stop(RenderProfiler.FINGERPRINT, shiftStart);
         if (plan == null) {
             return Set.of();
         }
@@ -692,6 +709,7 @@ final class TerminalPaneNode extends Region {
         private void render(RenderRow row) {
             prepareCanvas(row);
 
+            long drawStart = RenderProfiler.start();
             GraphicsContext gc = canvas.getGraphicsContext2D();
             gc.clearRect(0.0, 0.0, canvas.getWidth(), canvas.getHeight());
             gc.setFontSmoothingType(FontSmoothingType.LCD);
@@ -699,14 +717,20 @@ final class TerminalPaneNode extends Region {
 
             paintSidePadding(gc, row, canvas.getWidth(), canvas.getHeight());
             drawRow(gc, row, rowTop(row), metrics.cellWidth(), metrics.lineHeight());
+            RenderProfiler.stop(RenderProfiler.DRAW, drawStart);
+
+            long fpStart = RenderProfiler.start();
             cellFingerprints = cellFingerprints(row);
+            RenderProfiler.stop(RenderProfiler.FINGERPRINT, fpStart);
         }
 
         private void renderChanged(RenderRow row) {
             double oldWidth = canvas.getWidth();
             double oldHeight = canvas.getHeight();
             prepareCanvas(row);
+            long fpStart = RenderProfiler.start();
             long[] nextFingerprints = cellFingerprints(row);
+            RenderProfiler.stop(RenderProfiler.FINGERPRINT, fpStart);
             if (cellFingerprints.length != nextFingerprints.length
                     || oldWidth != canvas.getWidth()
                     || oldHeight != canvas.getHeight()) {
@@ -714,6 +738,7 @@ final class TerminalPaneNode extends Region {
                 return;
             }
 
+            long drawStart = RenderProfiler.start();
             GraphicsContext gc = canvas.getGraphicsContext2D();
             gc.setFontSmoothingType(FontSmoothingType.LCD);
             gc.setFont(metrics.font());
@@ -741,6 +766,7 @@ final class TerminalPaneNode extends Region {
             if (runStart >= 0) {
                 repaintColumns(gc, row, runStart, runEnd);
             }
+            RenderProfiler.stop(RenderProfiler.DRAW, drawStart);
             cellFingerprints = nextFingerprints;
         }