Add FPGA project workspace layout

FPGA/vivado/README.md

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+# FPGA/vivado/
+
+The Vivado project workspace. The committed source of truth is
+`create_project.tcl`; the project file (`FPGA-Core.xpr`) and all of Vivado's
+generated state (`*.runs/`, `*.cache/`, `*.hw/`, `*.sim/`, `*.ip_user_files/`,
+`*.gen/`, `*.srcs/`) are git-ignored — `.xpr` files embed machine-specific
+paths and are regenerated locally.
+
+RTL and testbench files are added to the project as **references** to
+`../rtl/...` / `../tb/...` so the actual source of truth stays in version
+control, not inside the project file.
+
+To generate the project, run from the repo root:
+
+```sh
+vivado -mode batch -source FPGA/vivado/create_project.tcl
+```
+
+Then open the resulting `FPGA/vivado/FPGA-Core.xpr` in the Vivado GUI.