Add the Sillitoe SDMS as a second mastering lathe candidate

Sillitoe Audio Technology (Australia) sells a complete cutting system:
lathe, MMFB feedback cutterhead, cutting amplifier, Space / Time Control
automation with a virtual groove, vacuum and microscope, at a published
USD 125,000 ex tax and shipping. Vendor landscape gets the narrative and
an Agnew-or-Sillitoe comparison; supplier map, total investment basis,
roadmap item 13 and the digital factory's no-telemetry section (STC cue
sheet, Open Groove export, stylus hours) follow.

docs/roadmap.md

@@ -61,7 +61,8 @@ These three run in parallel and gate everything downstream:
     certificate issuance
 12. Secure the site (lease preferred for the phased entry; purchase only
     with land privileges under BOI)
-13. Order long-lead items: mastering lathe (Agnew build times are long),
+13. Order long-lead items: mastering lathe (Agnew or Sillitoe, both built
+    to order, see [vendor landscape](suppliers/vendor-landscape.md)),
     stamper-prep machines, first utility packages
 14. Engage Stamper Discs for the bridge-stamper relationship with written
     turnaround SLAs