make the newcomer setup path work

Prerequisites named only Python and Blender while the install step sat 29 lines
below in Commands, so a new contributor hit a missing package before finding the
command that installs it. Install now sits inside Prerequisites, with the extras,
the flat requirements.txt list, and which commands actually need Blender. The
duplicate install block left Commands.

requirements.txt gives the flat dependency set in one file; pyproject extras stay
the source of truth.

Verified: README 3770 words against its frozen 3965 ceiling, the 238 words freed
by moving the probe retraction's tables to the docs that already own them;
doc_lint default and --all exit 0.

requirements.txt

@@ -0,0 +1,18 @@
+#   python -m pip install -r requirements.txt
+#   python -m pip install -e .            # the `orisys-design` entry point
+#
+# Flat instead of `-e .[dev,preview,optics,viz,probe]`, which needs network.
+
+PyYAML>=6.0
+pytest>=7.0
+numpy>=1.24
+PyQt5>=5.15
+PyOpenGL>=3.1
+matplotlib>=3.7
+opencv-python>=4.8
+
+# tests/test_uv_map.py imports `orisys.unroll.geometry` with no importorskip guard, so the
+# suite also needs the sibling repo: pip install -e ../orisys-sdk
+#
+# Blender is not a pip dependency: install the version in blender.lock.json, on PATH or via
+# ORISYS_BLENDER. Only the in-scene commands need it.