[timonix]

Why
The yosys toolchain could not build for Gowin FPGAs: nextpnr-himbaechel needs a CST constraint
file and writes its result as JSON, and gowin_pack produces an .fs bitstream. None of these
formats were supported.

What 1.
CST changes

New constraint file type cst. Like ccf, it is generated from the project's PCF file (which
the pin planner writes) into build/constraints.cst and passed to nextpnr as -o cst=....

ConstraintFileHelper now shares one PCF conversion between CCF and CST. Only the constraint
line format and the comment marker differ (# for CCF, // for CST). CCF output is
byte-identical to before. The PCF check and the "Converting ..." message moved into a small
helper in YosysService that both cases use.

API change: ConstraintFileHelper.Convert is got split into to ConvertPcfToCcf, next to the new
ConvertPcfToCst. It is public in the OneWare.OssCadSuiteIntegration package; I found no
callers outside this module. The old one still works, but I added a deprecated flag, in case there are extensions using it.

What 2.
JSON, .fs changes

These were already supported by yosys and oss cad suite, but were not possible to access.

• New place & route output type json, written with --write build/pnr.json and used as the
  pack tool's input.

• New pack output format fs (build/pack.fs). gowin_pack takes the output file as
  -o <file>, while icepack and ecppack take it as the second argument, so -o is only added
  for gowin_pack.

• cst, json and fs are selectable in the Toolchain Settings dialog and documented in the
  fpga-toolchain-yosys skill. openFPGALoader already supported fs.

Example board settings (Tang Nano 9K)

yosysToolchainYosysSynthTool:     synth_gowin
yosysToolchainNextPnrTool:        nextpnr-himbaechel
yosysToolchainNextPnrFlags:       --device GW1NR-LV9QN88PC6/I5 --vopt family=GW1N-9C
yosysToolchainConstraintFileType: cst
yosysToolchainOutputType:         json
yosysToolchainPackTool:           gowin_pack
yosysToolchainPackFlags:          -d GW1N-9C
packToolOutputFormat:             fs

The TangNano9K package 0.4 ships these defaults.