Add structural calculation worksheets

Collection of engineering calculation projects (Python + Typst), each with
input, calc script, tests, results, and generated PDF where available.

concentric-footing/input.yaml

@@ -0,0 +1,19 @@
+project: "Blavatnik"
+prepared_by: "Conemco Engineering"
+
+Ps: "18.4 kip"           # service axial (checked demand)
+Pu: "26.2 kip"           # factored axial (checked demand, 1.2D+1.6L)
+qa: "2500 psf"           # allowable soil bearing (gross)
+Bf: "3 ft"               # square footing side
+Df: "12 in"              # total thickness
+cover: "3 in"            # to centroid of steel -> d = Df - cover
+fc: "3000 psi"           # f'c
+fy: "60 ksi"             # fy
+lambda: 1                # lightweight factor (float, (0,1])
+column_width: "14 in"    # c, square column
+base_plate_width: "6 in" # bp, square base plate (A1 = bp^2); if omitted in code defaults to c
+N: 4                     # bars per direction
+rebar_size: 4            # #4 -> db=0.5in
+
+# Optional documentation keys (not used in compute, but keep for Typst derivation comments):
+# DLr, LLr etc are NOT in input.yaml; Typst derives Ps/Pu there. This YAML holds checked demands only.