Close pass four: undetented host curve read, LiveValues guard, ordering assert, static-lane fix, docs

Points toPlain's exponent arm at the undetented curve map so host reads
match the editor; adds a sizeof guard plus field-poison test for
LiveValues::operator==; skips the model write when an automation value
hasn't moved; corrects five stale doc citations.

src/core/instrument/engine/live_params.h

@@ -70,6 +70,13 @@ struct LiveValues {
 // The seqlock copies the block as raw bytes, which is only defensible for a plain value type.
 static_assert(std::is_trivially_copyable_v<LiveValues>,
               "the live block is copied under a seqlock — it must stay a plain value");
+// Guards operator== against silent staleness: a member added to the struct above changes this
+// size, so the assert fails at the new member's own commit instead of leaving a live control
+// that never reaches a sounding voice with no compiler or test signal. Confirmed 352 bytes,
+// MSVC 19.44 x64, Release (`SizeProbe<sizeof(LiveValues)>`, an incomplete-template size probe
+// whose error message reports the value). Bump the literal AND operator== together.
+static_assert(sizeof(LiveValues) == 352,
+              "a member was added or removed — extend operator== in live_params.cpp to match");
 
 // FIELD-wise equality, and it must never be "simplified" into a memcmp. LiveValues carries
 // padding, and nothing gives that padding a determinate value across a copy: NRVO is optional