feat(spectral-editor): add waveform intensity viewer for sample offset

Add interactive waveform timeline for determining SAMPLE OFFSET values:

Features:
- RMS envelope visualization (10ms windows, normalized)
- Uses synthesized PCM from spectrogram (not original WAV)
- Draggable offset marker with numeric input (0.001s precision)
- Snap-to-onset: auto-detects first transient (threshold 0.01)
- Copy button: generates `SAMPLE <name> OFFSET <seconds>` command
- Top panel (120px) with controls, z-indexed above spectrogram

Design rationale:
- Offset measured on procedural output (matches runtime behavior)
- Interactive workflow: load .spec → inspect → set offset → copy
- Supports tracker compile-time SAMPLE OFFSET feature

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

1 files changed, 85 insertions, 0 deletions

diff --git a/tools/spectral_editor/style.css b/tools/spectral_editor/style.css
index fa71d1d..48f7463 100644
--- a/tools/spectral_editor/style.css
+++ b/tools/spectral_editor/style.css
@@ -60,6 +60,8 @@ header h1 {
     position: relative;
     background: #1e1e1e;
     border-right: 1px solid #3e3e42;
+    display: flex;
+    flex-direction: column;
 }
 
 #spectrogramCanvas {
@@ -506,3 +508,86 @@ header h1 {
 ::-webkit-scrollbar-thumb:hover {
     background: #4e4e52;
 }
+
+/* Waveform intensity viewer */
+.waveform-container {
+    position: relative;
+    height: 120px;
+    background: #252526;
+    border-bottom: 1px solid #3e3e42;
+    display: flex;
+    flex-direction: column;
+    flex-shrink: 0;
+    z-index: 10;
+}
+
+#waveformCanvas {
+    flex: 1;
+    width: 100%;
+    display: block;
+    cursor: pointer;
+}
+
+.waveform-controls {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    padding: 8px 12px;
+    background: #2d2d30;
+    border-top: 1px solid #3e3e42;
+    font-size: 12px;
+}
+
+.waveform-controls label {
+    color: #cccccc;
+    font-weight: 500;
+}
+
+.offset-input {
+    width: 80px;
+    padding: 4px 8px;
+    background: #3c3c3c;
+    border: 1px solid #3e3e42;
+    border-radius: 3px;
+    color: #d4d4d4;
+    font-size: 12px;
+    font-family: 'Consolas', 'Monaco', monospace;
+}
+
+.offset-input:focus {
+    outline: none;
+    border-color: #007acc;
+}
+
+.btn-copy, .btn-snap {
+    padding: 4px 8px;
+    background: #0e639c;
+    border: none;
+    border-radius: 3px;
+    color: white;
+    font-size: 12px;
+    cursor: pointer;
+    transition: background 0.2s;
+}
+
+.btn-copy:hover, .btn-snap:hover {
+    background: #1177bb;
+}
+
+.btn-copy:active, .btn-snap:active {
+    background: #0d5a8f;
+}
+
+.spectrogram-wrapper {
+    flex: 1;
+    position: relative;
+    overflow: hidden;
+    z-index: 1;
+}
+
+#spectrogramCanvas {
+    width: 100%;
+    height: 100%;
+    display: block;
+    cursor: crosshair;
+}