[Copilot]

CI should not depend on the efabless/openlane Docker image. This PR adds a fast Yosys synthesis check in its place and documents upstream OpenLane for full P&R, which stays a local/manual step.

• CI (.github/workflows/sky130-smoke.yml)

  • The file did not exist, so it is created with sim, formal and a new synthesis-check job.
  • synthesis-check runs make sky130-synth-yosys-only.
  • The efabless Docker smoke test is dropped, not kept as optional.
  • The formal job's sby install step uses || true. A failed install only surfaces when make formal runs.

• Makefile

  • sky130-synth-yosys-only runs PDK-agnostic synth and check -assert, with no liberty file or Docker.
  • sky130-synth-local does dfflibmap and abc against the sky130_fd_sc_hd liberty. Set SKY130_LIB, or PDK_ROOT to locate it. It writes build/synth/stat.txt and a mapped netlist, and fails with a clear message if the liberty is missing.
  • sky130-synth now invokes upstream OpenLane through OPENLANE_ROOT and is marked as requiring Docker.
  • clean also removes build/.
  • The frozen widths (DATA_W=8, ACC_W=16, LEN_W=6) are applied with chparam, because the RTL defaults are still 24 and 8.

• Docs

  • openlane/README.md covers the CI (Yosys) vs. local (OpenLane) split and how to run upstream OpenLane locally.
  • docs/SYNTHESIS.md covers the Yosys flow with sky130 .lib/.lef, the commands, and expected cell mappings and constraints.
  • The root README.md gets a "Physical Design Flow" section and updated file list.

make sky130-synth-yosys-only                  # CI: no PDK, no Docker
make sky130-synth-local PDK_ROOT=$HOME/.volare  # sky130_fd_sc_hd mapping

Review notes

• The repo has no OpenLane config.json, so make sky130-synth will not run end to end yet. openlane/README.md only lists what it must contain (top dpu_tile, the frozen widths, 10 ns clock).
• The CI check is PDK-agnostic, so it does not validate sky130 mapping or timing. Those happen locally via sky130-synth-local and OpenLane.
• I only ran sky130-synth-yosys-only, which passes (754 cells). sky130-synth-local and sky130-synth have not been run, and the workflow has not run in CI.