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/common.py

@@ -0,0 +1,115 @@
+"""Shared constants and small layout helpers for the viewer widgets."""
+
+from PyQt6.QtCore import Qt
+from PyQt6.QtWidgets import (
+    QFormLayout, QFrame, QGroupBox, QLabel, QScrollArea, QSizePolicy,
+    QVBoxLayout, QWidget,
+)
+
+from sras_format import CH1_IDX, CH3_IDX, CH4_IDX
+
+# ---------------------------------------------------------------------------
+# Display constants
+# ---------------------------------------------------------------------------
+
+CH_LABELS = [
+    "CH1 — RF (FFT peak freq)",
+    "CH3 — Bias A (DC mean)",
+    "CH4 — Bias B (DC mean)",
+    "CH1 — Velocity (SRAS)",
+]
+
+# Combo index for the derived velocity mode (uses CH1_IDX data)
+VELOCITY_MODE_IDX = 3
+# All modes that operate on CH1 waveforms
+CH1_DERIVED_MODES = (CH1_IDX, VELOCITY_MODE_IDX)
+
+CMAPS = ["gray", "viridis", "plasma", "inferno", "hot", "jet", "RdBu_r", "seismic"]
+
+# (mode_str, status-bar unit, colorbar label) per channel index
+_CHANNEL_DISPLAY = {
+    CH1_IDX: ("RF", "Peak frequency (MHz)", "MHz"),
+    CH3_IDX: ("DC", "DC mean (mV)", "mV"),
+    CH4_IDX: ("DC", "DC mean (mV)", "mV"),
+    VELOCITY_MODE_IDX: ("Velocity", "Velocity (m/s)", "m/s"),
+}
+
+_CSS_HINT = "font-size: 11px; color: #aaa;"
+_CSS_INFO = "font-size: 11px;"
+_CSS_MUTED = "color: #888; font-size: 11px;"
+_CSS_WARN = "color: #e07000; font-size: 11px;"
+_CSS_BUSY = "color: #4a90d9; font-size: 11px;"
+
+# Side-panel column widths (the scroll areas that hold the controls).
+_LEFT_PANEL_W = 288
+_RIGHT_PANEL_W = 272
+
+# Minimum width for a spin box so its value + suffix are never clipped.
+_SPIN_MIN_W = 96
+
+
+# ---------------------------------------------------------------------------
+# Small layout helpers
+# ---------------------------------------------------------------------------
+
+def _wrap_label(text: str = "", css: str | None = None) -> QLabel:
+    """A word-wrapped QLabel that reports its *wrapped* height to the layout.
+
+    A plain word-wrapped QLabel advertises a single-line minimum height, so in a
+    fixed-width column the layout happily shrinks it and the extra lines get
+    clipped.  Enabling height-for-width makes the box layout ask for the real
+    height at the column's width instead.
+    """
+    lbl = QLabel(text)
+    lbl.setWordWrap(True)
+    sp = lbl.sizePolicy()
+    sp.setVerticalPolicy(QSizePolicy.Policy.Minimum)
+    sp.setHeightForWidth(True)
+    lbl.setSizePolicy(sp)
+    if css:
+        lbl.setStyleSheet(css)
+    return lbl
+
+
+def _group(title: str) -> tuple[QGroupBox, QVBoxLayout]:
+    """A group box with consistent, non-cramped internal margins."""
+    grp = QGroupBox(title)
+    lay = QVBoxLayout(grp)
+    lay.setContentsMargins(10, 8, 10, 10)
+    lay.setSpacing(6)
+    return grp, lay
+
+
+def _form() -> QFormLayout:
+    """A label/field form layout for a narrow side panel."""
+    form = QFormLayout()
+    form.setContentsMargins(0, 0, 0, 0)
+    form.setHorizontalSpacing(8)
+    form.setVerticalSpacing(6)
+    form.setLabelAlignment(Qt.AlignmentFlag.AlignRight
+                           | Qt.AlignmentFlag.AlignVCenter)
+    form.setFormAlignment(Qt.AlignmentFlag.AlignLeft | Qt.AlignmentFlag.AlignTop)
+    form.setFieldGrowthPolicy(
+        QFormLayout.FieldGrowthPolicy.AllNonFixedFieldsGrow)
+    form.setRowWrapPolicy(QFormLayout.RowWrapPolicy.DontWrapRows)
+    return form
+
+
+def _scroll_panel(inner: QWidget, width: int) -> QScrollArea:
+    """Put a side panel in a fixed-width scroll area.
+
+    Without this the panels are sized by the window: a short window squeezes the
+    controls past their minimum heights, which is what makes text overlap the
+    widget below it.  Scrolling keeps every control at its natural size.
+    """
+    area = QScrollArea()
+    area.setWidget(inner)
+    area.setWidgetResizable(True)
+    area.setFrameShape(QFrame.Shape.NoFrame)
+    area.setHorizontalScrollBarPolicy(Qt.ScrollBarPolicy.ScrollBarAlwaysOff)
+    area.setVerticalScrollBarPolicy(Qt.ScrollBarPolicy.ScrollBarAsNeeded)
+    area.setFixedWidth(width)
+    area.viewport().setAutoFillBackground(False)
+    inner.setAutoFillBackground(False)
+    return area
+