Add the physics material density database

physics_materials.yaml

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+# Densities of materials in kg/m^3.
+# FIXME: friction belongs per pair of materials; genesis-world resolves `max(mu_a, mu_b)`
+# until it supports pairs.
+# --- polymers ---
+abs_plastic:
+  rho: 1040.0 # ABS (MatWeb, injection-moulding grades)
+polypropylene:
+  rho: 920.0 # PP homopolymer (MatWeb)
+hdpe:
+  rho: 950.0 # High-density polyethylene (MatWeb)
+acrylic_pmma:
+  rho: 1180.0 # Cast PMMA (MatWeb)
+nylon:
+  rho: 1140.0 # Nylon 6/6, unfilled (MatWeb)
+silicone_rubber:
+  rho: 1200.0 # Unfilled silicone elastomer (MatWeb)
+# --- ceramics and glass ---
+stoneware_ceramic:
+  rho: 2300.0 # Glazed stoneware (Ceramic Industry handbook)
+porcelain:
+  rho: 2400.0 # Vitrified porcelain (Ceramic Industry handbook)
+glass:
+  rho: 2500.0 # Soda-lime glass (Corning)
+# --- metals ---
+aluminium:
+  rho: 2700.0 # Al 6061-T6 (MatWeb)
+steel:
+  rho: 7850.0 # Low-carbon steel (MatWeb)
+stainless_steel:
+  rho: 7900.0 # AISI 304 (MatWeb)
+# --- wood and paper ---
+pine:
+  rho: 500.0 # Softwood at 12% moisture (Wood Handbook Tab. 4-3)
+oak:
+  rho: 750.0 # Red oak at 12% moisture (Wood Handbook Tab. 4-3)
+plywood:
+  rho: 600.0 # Softwood plywood (APA Engineered Wood Handbook)
+cardboard:
+  rho: 700.0 # Solid-fibre paperboard (TAPPI)
+paper:
+  rho: 800.0 # Uncoated office paper (TAPPI)
+# --- moulded replicas ---
+# Bulk density over the solid collision volume. A replica is a hollow shell, so this sits far below
+# the polymer it is moulded from.
+plastic_produce:
+  rho: 100.0 # Geometric mean of the seven produce twins weighed in #3638, which span 50-160