Split sras_viewer.py into a package (pure move)

The 2790-line module becomes sras_viewer/: common.py (constants +
layout helpers), canvases.py (RoiQuad, ImageCanvas, WaveformCanvas,
ManualAlignOverlayCanvas), dialogs.py (FftOptionsDialog,
ManualAlignmentDialog), main_window.py (SrasViewerWindow + main), with
__init__ re-exporting the public names and __main__ keeping
`python -m sras_viewer` working. pyproject gains a `sras-viewer`
console script.

Code moved verbatim; only import headers are new (pyflakes-clean).
tests/test_gui.py patch targets follow the classes to their new
modules.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

sras_viewer/__init__.py

@@ -0,0 +1,24 @@
+"""
+SRAS Scan File Viewer
+PyQt6 application for visualizing channel data from .sras binary scan files.
+
+Channel semantics (fixed by sc3_aui_app.py acquisition settings):
+  CH1 — RF Acoustic Packet (AC-coupled, 100 mV/div): FFT → peak frequency
+  CH3 — Bias A (DC-coupled, 50 mV/div): waveform mean
+  CH4 — Bias B (DC-coupled, 50 mV/div): waveform mean
+
+RF images are masked: pixels where CH4_dc < dc_threshold show 0.
+
+File parsing lives in sras_format, image/alignment math in sras_compute, and
+background workers in sras_workers — none of which import Qt or matplotlib,
+so multiprocessing children can load them cheaply.
+"""
+
+import faulthandler
+
+faulthandler.enable()   # print a native stack trace on SIGSEGV/SIGABRT/etc.
+
+from .canvases import ImageCanvas, RoiQuad, WaveformCanvas           # noqa: E402,F401
+from .common import CH_LABELS, CMAPS, VELOCITY_MODE_IDX              # noqa: E402,F401
+from .dialogs import FftOptionsDialog, ManualAlignmentDialog         # noqa: E402,F401
+from .main_window import SrasViewerWindow, main                      # noqa: E402,F401