docs: anchor the product-doc citations to symbols, not line numbers

Ten more were already pointing at unrelated code. Vendor SDK and SWELL citations left alone — those files don't move.

docs/product/midi-playback.md

@@ -1117,7 +1117,7 @@ Addendum is the *why*; those are the *what/how*.
 
 **Framing.** Folds one more control into the S-VIEW redesign: a **visual velocity → amp
 transfer-curve editor**. Today the engine maps velocity to gain *linearly* (`velocityGain_ =
-velocity / 127.0`, `sampler_core.cpp:261`), applied once at note-on in `Voice::start()`. Daniel
+velocity / 127.0`, `Voice::start()`), applied once at note-on in `Voice::start()`. Daniel
 wants that mapping to become an **editable transfer curve** — a bezier from a default flat line to
 an arbitrary multi-point curve — so velocity dynamics are fully shapeable per sound.
 
@@ -1181,7 +1181,7 @@ a LICE shell that draws handles and routes the mouse).
    evaluation is called at note-on, not per frame (see call 4).
 
 4. **Voice-engine application point → `Voice::start()`, replacing the linear `velocity/127`.**
-   Confirmed from source: `sampler_core.cpp:261` computes `velocityGain_ = velocity / 127.0` **once
+   Confirmed from source: `Voice::start()` computes `velocityGain_ = velocity / 127.0` **once
    at note-on** inside `Voice::start()`; the per-frame render path (`advanceFrame`, line 408:
    `gain = amp * velocityGain_`) then just multiplies the cached scalar. So the transfer curve
    slots in at exactly one line: `velocityGain_ = curve.eval(velocity)` at note-on — **off the