synth: blackbox SYNTH_BLACKBOXES modules before hierarchy check

[oharboe]

Re-opening of #4211 after measuring the cost of the alternative.

Summary

Adds a checkpoint-based code path for SYNTH_BLACKBOXES to synth.tcl and aligns the existing source-based path in synth_preamble.tcl so all flows behave identically (use catch, blackbox before hierarchy -check -top).

Why this is needed: parallel partition synthesis with slang

When --keep-hierarchy is used (required for partition flows that preserve internal hierarchy), the slang frontend mangles module names by their elaboration path so different parameterizations get distinct names — Foo instantiated at top.unit.foo becomes Foo$top.unit.foo in the elaborated RTLIL. The parent design references those mangled names.

A "per-partition canonicalize from RTL" approach therefore can't just elaborate Foo as a fresh top — that would emit a module named Foo, but the parent design instantiates Foo$top.unit.foo, and the link fails.

The orchestrator pattern instead is: canonicalize the full design once (so all parameterized instances get their elaborated names), and each partition then loads that shared checkpoint and blackboxes the modules outside its scope. Every partition reads the same canonical RTLIL and only synthesises its own subhierarchy.

The same SYNTH_BLACKBOXES list can be reused across all partitions because unknown names are skipped silently with catch { blackbox $m } instead of erroring.

Cost evidence

Measured on an test case I can't share (46 partitions, ~3,500 s of synth CPU compressed into ~5 min wall via 12× parallelism): the shared canonicalize is 7 s (read_liberty 2 s + opt_clean 1.9 s + read_slang 1.5 s + misc) — necessary for slang to consistently mangle parameterised names across partitions, not optional bookkeeping.

Test plan

• No behaviour change when SYNTH_BLACKBOXES is unset
• DCO + Tclint + variables.yaml tests pass on the previous PR (synth: blackbox SYNTH_BLACKBOXES modules before hierarchy check #4211) iteration
• CI green

[gemini-code-assist]

Code Review

This pull request introduces support for blackboxing modules during synthesis to facilitate parallel synthesis partitions. It adds logic to synth.tcl and synth_preamble.tcl to iterate through SYNTH_BLACKBOXES and apply the blackbox command with error handling. Feedback was provided regarding synth_preamble.tcl, noting that the hierarchy command should be moved outside the conditional block to ensure the design is elaborated regardless of whether blackboxing is performed.

[oharboe]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces logic to blackbox modules specified in SYNTH_BLACKBOXES when loading a design from an RTLIL checkpoint in flow/scripts/synth.tcl, supporting parallel synthesis flows. Feedback suggests improving the blackboxing mechanism to handle module name mangling (e.g., from the slang frontend) using pattern-based selection. Additionally, the PR description should be updated to resolve a contradiction with the code comments regarding the harmonization of this logic with synth_preamble.tcl.