DFlash2: STRATA_DF2_END_UPDATE=0 turns the end-of-request context update off (diagnostics)

src/program/generate.cpp

@@ -17841,7 +17841,8 @@ int main(int argc, char** argv) {
 #if defined(STRATA_HAVE_DFLASH2)
                 // The request ends here, so no draft follows and the window's committed rows would never reach the drafter's ring or its log: every turn would
                 // leave a hole of a + 1 positions behind it, which a later turn that resumes after this one finds blank (and cannot rebuild). Update them now.
-                if (use_df2 && req_df2 && !merged && (eos || produced_n >= max_new)) {
+                static const bool df2_end_update = std::getenv("STRATA_DF2_END_UPDATE") == nullptr || std::getenv("STRATA_DF2_END_UPDATE")[0] != '0';   // =0: the pre-fix behaviour (diagnostics)
+                if (df2_end_update && use_df2 && req_df2 && !merged && (eos || produced_n >= max_new)) {
                     if (!df2.context(ver.tap_dev(), a + 1, p, err)) { if (adapt_thr.joinable()) adapt_thr.join(); std::printf("ERR %s\n", err.c_str()); return 1; }
                 }
 #endif