Add marking method per material: cold scribe on lacquers, laser on nickel, none on PVC

Laser marking excluded on lacquers (nitrocellulose flammability) and on
pressed records (HCl fumes from PVC), per the safety commitments

docs/quality-automation.md

@@ -45,7 +45,7 @@ batch ID and writes to the genealogy database.
 
 ### Part marking and orientation fiducials
 
-Every metal part gets machine-readable identity, using two complementary
+Every metal part gets a machine-readable identity, using two complementary
 marks:
 
 - **Dead-wax marks**: matrix codes scribed in the runout area are
@@ -61,6 +61,24 @@ marks:
   marked its stamper backs. This mark does not propagate to records; it
   pairs with the dead-wax identity.
 
+**Marking method per material** (safety-driven, not preference):
+
+- **Lacquers: mechanical cold scribe only, never laser.** Nitrocellulose is
+  highly flammable and thermally sensitive, and the lacquer is the audio
+  master; a stylus scribe in the dead wax is the century-proven method and
+  can be automated with a small CNC scriber if machine-consistent marking
+  is wanted. The lacquer is destroyed when the father is separated, so its
+  identity lives on through the propagated scribe, not the part.
+- **Nickel parts: laser marking, freely.** Fiber-laser serials and Data
+  Matrix codes on the backs, per optical-disc practice.
+- **Pressed records: no laser marking.** Laser-burning PVC releases
+  hydrochloric acid fumes, a worker-safety and equipment-corrosion problem.
+  Records already carry the pressed-in dead-wax marks from the stamper, and
+  per-record uniqueness comes from the traceability database (press, cycle,
+  run position) rather than physical marks.
+- **Labels and packaging: printed or inkjet codes**, which the packaging QC
+  barcode checks already assume.
+
 The galvanic process itself is rotationally symmetric, so orientation never
 needs to be locked through the baths, only tracked, and the fiducial does
 that. The fiducial also gives the inline optical inspection a fixed angular
@@ -72,8 +90,8 @@ committing to it.
 
 ### Industry context
 
-Even a respected stamper house checks thickness with a
-hand micrometer at 10 points on sacrificial blanks, with nothing logged.
+Even a respected stamper house checks thickness with a hand micrometer at
+10 points on sacrificial blanks, with nothing logged.
 Automated non-contact metrology with per-serial logging is ahead of current
 industry practice, which is exactly the point: it makes every father, mother,
 and stamper a data-backed accept/reject decision before the next plating