build: Phase 2 — stop emitting unused .blockmap + clean dist per build

- L83: nsis.differentialPackage: false — the custom updater does full
  downloads and never consumes the differential blockmap, so don't generate it
  (NsisTarget gates blockmap generation on differentialPackage !== false).
- L84: add clean:dist script (fs.rmSync) run at the front of build:win so dist/
  no longer accumulates old-version installers (~3 GB observed). dist/out were
  already gitignored, so this is local-disk hygiene.

Config only; TS gate unaffected. 137 -> 135 open audit items.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

electron-builder.yml

@@ -68,4 +68,7 @@ nsis:
   perMachine: false
   allowToChangeInstallationDirectory: true
   deleteAppDataOnUninstall: false
+  # The custom updater (src/main/updater.ts) does a full download and never consumes
+  # the differential .blockmap, so don't emit it (audit L83 — dead build artifact).
+  differentialPackage: false
   # Flip perMachine to true for an all-users install to Program Files (requires admin).