fix: seek lower-bound guard and pointer-down callback ordering

AudioPlayer.ts: route seeks below bufferStart to seekBeyondBuffer;
previous missing lower-bound caused clamped playback after first seek.
WaveformSeeker: fire OnSeekStart/OnSeekChange before capturePointer
await to prevent fast-click race that locked _isSeeking true.
Latent: WavOffsetService encodes remaining-only DataSize, overwriting
JS this.duration after seek — not fixed here, scope separately.

DeepDrftPublic/Interop/audio/AudioPlayer.ts

@@ -274,14 +274,18 @@ export class AudioPlayer {
             return { success: false, error: 'Invalid seek position' };
         }
 
-        // Get buffered duration (accounting for playback offset)
-        const bufferedDuration = this.scheduler.getTotalDuration() + this.scheduler.getPlaybackOffset();
+        const bufferStart = this.scheduler.getPlaybackOffset();
+        const bufferEnd = this.scheduler.getTotalDuration() + bufferStart;
 
-        // Check if seeking within buffered content
-        if (position <= bufferedDuration) {
+        // Position must be within [bufferStart, bufferEnd] to use buffered content.
+        // A lower-bound check is required: after a seek-beyond-buffer, bufferStart is
+        // set to the prior seek position. Seeking to a position below bufferStart would
+        // produce a negative bufferRelativePosition in seekWithinBuffer, silently
+        // clamping to position 0 of the offset buffer instead of the requested time.
+        if (position >= bufferStart && position <= bufferEnd) {
             return this.seekWithinBuffer(position);
         } else {
-            // Seeking beyond buffer - signal C# to fetch new stream
+            // Seeking outside buffered window - signal C# to fetch new stream
             return this.seekBeyondBuffer(position);
         }
     }