feat(S9/S8): bank-generation change-detection + instrument-side assignment reader

Extension stamps a monotonic bank_generation counter, bumped at content
mutations; the VST3 instrument polls it off-thread on an editor timer and
consumes assignment requests, refreshing playback hands-free.

src/ext_keys.h

@@ -45,6 +45,19 @@ inline constexpr const char* kProjExtTailKey = "tail_setting";
 // The per-project minted-GUID identity key.
 inline constexpr const char* kProjExtGuidKey = "project_guid";
 
+// The S9 BANK-GENERATION key. The EXTENSION stamps a monotonic decimal counter here that it
+// bumps on every bank-content mutation that changes what a live instance would PLAY (capture
+// add, re-capture-in-place, sample remove, move/copy affecting banks, ingest import). The VST3
+// instrument READS it off the audio thread on a UI-timer cadence and, when the value differs
+// from what it last saw, calls reloadFromBank() so a recapture/ingest refreshes playing
+// instances hands-free (the S9 change-detection trigger). WIRE-SHARED (instrument reads it);
+// the instrument never WRITES it (the extension owns it, same read-only-over-bank rule as the
+// assignment request). Additive to the persist blob — an absent stamp reads as generation 0
+// (a pre-S9 project), and the first bump (>= 1) then reads as a change. FOREVER-STABLE once
+// shipped: changing this spelling resets every already-shipped instance's change-detection
+// baseline (a one-time spurious reload), so it is fixed like every sibling key.
+inline constexpr const char* kProjExtBankGenKey = "bank_generation";
+
 // The S8 ingest ASSIGNMENT-REQUEST key. The EXTENSION writes an assignment request here
 // after an ingest-with-assign (arrange capture / Media-Explorer import / drop-onto-panel):
 // "the active sampler instance should now play THIS sample." The value is the pure