fix(zoom): drive pinch absolutely from gesture-start snapshot; snappier pen

Zoom flicker root cause (from the on-device badnote_input_log.txt): the pinch
computed its per-frame scale change as desiredScale / getMaxScaleOnAxis(), i.e.
it fed the LIVE matrix back into its own update. Consecutive frames in the log
show `cur` (the live read) dropping to 0.75-0.89 for a single frame while the
result track stayed smooth, so the code demanded a 1.3-1.4x correction that
popped the zoom bigger/smaller and snapped back. The >1.4 glitch guard missed it
because the spikes sat at 1.31-1.40.

Fix: the scale branch of PenInteractiveViewer now drives the transform
ABSOLUTELY from a gesture-start snapshot (_scaleStart, _referenceFocalPoint) plus
the recognizer's clean, monotonic cumulative details.scale. Each frame is fully
re-derived in closed form (pure scale+translate, no matrix inversion, no live
read-back), so a transient mis-read or interleaved write cannot survive into the
next frame. The per-frame glitch guard now keys on the recognizer's own
scale-ratio (the true finger motion) instead of the corrupted live read. 2-finger
pan still falls out of the same focal-anchor formula.

Pen feel: lower perfect_freehand streamline 0.5 -> 0.32 (new shared constants
kPenStreamline/kPenSmoothing, single-sourced across screen + export so the
parity test still holds). At 0.5 a quick flick lagged so far behind the pen that
short fast strokes collapsed toward their start and rendered as a dot
("写字识别成单击"); 0.32 tracks the real path for a crisper, lower-latency feel.

flutter analyze lib/editor clean; 66/66 tests pass (incl. screen==export parity).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/editor/canvas/ink_painters.dart

@@ -8,7 +8,8 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:perfect_freehand/perfect_freehand.dart' as pf;
 
-import '../engine/stroke_geometry.dart' show kDefaultPenThinning;
+import '../engine/stroke_geometry.dart'
+    show kDefaultPenThinning, kPenSmoothing, kPenStreamline;
 import 'pen_stroke.dart';
 
 /// Builds a filled outline [Path] for one stroke (already scaled to pixels).
@@ -45,8 +46,8 @@ Path buildStrokePath(
       // Highlighter keeps a constant width (no thinning); pen uses the
       // configurable [thinning] so real Surface-Pen pressure changes width.
       thinning: isHighlighter ? 0.0 : thinning,
-      smoothing: 0.5,
-      streamline: 0.5,
+      smoothing: kPenSmoothing,
+      streamline: kPenStreamline,
       // Real stylus pressure → don't simulate; no pressure → let freehand fake
       // it based on velocity. (perfect_freehand 2.x honors REAL pressure when
       // simulatePressure is false — 1.0.4 ignored it, which made width