Load FSDB scopes lazily and nest struct fields

[amikhalev]

Summary

• Keep a compact native FSDB scope index during ffrReadScopeTree instead of eagerly building millions of NetlistItems over IPC (avoids extension-host OOM on large dumps).
• Load child scopes on expand via getScopeChildren, then vars via ffrReadVarByLogUOff (same on-demand pattern as VCD/FST).
• Emit FSDB STRUCT/RECORD as nested netlist scopes so leaf names like aw_ready are not flattened into duplicates under the parent module.

Test plan

• Rebuild FSDB addon (npm run compile-addon) with Verdi FSDB reader libs/headers
• Open a small FSDB and confirm hierarchy expands (modules → children → signals)
• Open a large multi-million-scope FSDB and confirm “Reading Scopes” completes without JS heap OOM
• Expand a scope that contains SV structs/AXI interfaces and confirm signals appear under struct folders (no repeated aw_ready at the parent level)
• Add a few signals to the viewer and confirm waveforms still load

[Lramseyer]

This does not need to be removed

[Lramseyer]

@heyfey - Can you review this? This looks (mostly) good from my end.

[heyfey]

Hi @amikhalev , thanks for your contribution! I will review it soon.

And yes it's a great idea to defer NetlistItem creation to when we actually need it. I built eveything in JS side to avoid duplicating tree data in JS & C++, but turn out it's really slow for JS to create millions of treeitems.

Your changes have one more benifit: by keeping a copy of scope tree in C++ side, we can easily implement "find scope/instance" there (#152) (I hesitate for "find signal" for fsdb)