synth: single writer for 1_synth.sdc, fail hard on missing outputs

[oharboe]

Root-cause fix for the read-only 1_synth.sdc failure hzeller hit on RBE (OpenROAD PR #10818 worked around it with chmod +w).

The bug

1_synth.sdc had two writers in ORFS:

1. yosys' synth.tcl copied the user's SDC_FILE into place — a leftover from before SDC canonicalization existed ("One day a more sophisticated synthesis will write out a modified .sdc").
2. synth_odb.tcl then overwrote it in place with the OpenSTA-canonicalized version (write_sdc -no_timestamp).

cp preserves mode bits, so with read-only inputs (bazel RBE) the copy is read-only and the canonicalization write fails. On writable local checkouts it only accidentally worked.

The fix

patches/0037 (carried until the identical fix lands in ORFS upstream, then dropped at the next bump):

• synth.tcl / synth_preamble.tcl no longer produce 1_synth.sdc; the yosys stage's SDC output is 1_2_yosys.sdc (raw copy), as before.
• synth_odb.tcl / synth_syn.tcl are the sole writers of 1_synth.sdc, and write 1_synth.odb/.sdc unconditionally: producing them is the do-1_synth contract. The WRITE_ODB_AND_SDC_EACH_STAGE=0 no-op silently left downstream stages consuming the stale raw copy.

rules.bzl (fail hard and early instead of festering):

• 1_synth.sdc is declared together with 1_synth.odb only when save_odb=True, instead of as a yosys output. (save_odb=False users only consume the 1_2_yosys.v output group.)
• Stop touch-masking primary artifacts (1_2_yosys.v/.sdc, 1_synth.odb/.sdc) after make: a synth step that silently produced nothing used to yield empty files that flowed downstream — and would let idempotency tests compare two empty files as "identical". Now bazel fails the action with "declared output was not created". Logs, reports and mem.json remain touch-stubbed since the SYNTH_NETLIST_FILES path legitimately skips them.

Testing

• bazelisk test //test/...: 43 pass; 17 fail to build locally due to the pre-existing @scip glibc 2.41 compile issue (unrelated — real-OpenROAD targets; covered by CI).
• Verified the patch applies to the pinned ORFS and that the patched @orfs scripts contain a single 1_synth.sdc writer.

Follow-ups after merge: bazelisk run @bazel-orfs//:bump in OpenROAD (unblocks the RBE tests, replaces #10818), and the same fix as an ORFS PR so patch 0037 can be dropped.

🤖 Generated with Claude Code

[oharboe]

@hzeller FYI