fix: set f_ref in SchematicComponentV6.set_ref() for KiCad 9 compat

[telagod]

Summary

• SchematicComponentV6.set_ref() sets self.ref but never self.f_ref
• When a KiCad 9 schematic component is missing one of the 4 basic fields (Reference/Value/Footprint/Datasheet), _solve_fields() in v5_sch.py:1073 accesses self.f_ref for the warning message
• This causes AttributeError: 'SchematicComponentV6' object has no attribute 'f_ref'
• KiCad 9 changed how fields are stored — fewer default fields in the instance — so this hits virtually all KiCad 9 schematics

The fix

One line: self.f_ref = ref in set_ref(), mirroring what the V5 parsing path does.

def set_ref(self, ref):
    self.ref = ref
    self.f_ref = ref    # <-- added
    self.split_ref()
    self.set_field('Reference', ref)

Traceback

File "kibot/kicad/v6_sch.py", line 3055, in load
    comp.set_ref(s.reference)
File "kibot/kicad/v6_sch.py", line 1269, in set_ref
    self.set_field('Reference', ref)
File "kibot/kicad/v5_sch.py", line 955, in set_field
    self._solve_fields(LineReader(None, '**Internal**'))
File "kibot/kicad/v5_sch.py", line 1073, in _solve_fields
    logger.warning(W_MISCFLD + 'Component `{}` without the basic fields'.format(self.f_ref))
                                                                                ^^^^^^^^^^
AttributeError: 'SchematicComponentV6' object has no attribute 'f_ref'

Test plan

• Tested with KiBot 1.9.0 + KiCad 9.0.9 on a 91-component board (ESP32-C3, HV boost, USB-C)
• Before fix: crash on bom, render_3d, erc, pdf_sch_print — any output that calls GS.load_sch()
• After fix: all pass. Warnings about "without the basic fields" are expected and harmless (W036)
• BOM CSV output is correct (92 lines, all components present)
• ERC preflight runs to completion (186 unique warnings, 0 errors)
• PCB-only outputs (gerber, drill, position) still work as before

[set-soft]

Hi @telagod !

1. You need to rebase this patch, use git rebase -i HASH_FOR_THE_LAST_PATH_IN_DEV and let just the last entry.
2. When you say KiCad 9 I guess you are talking about KiCad 9.0.9, right? I have 9.0.8 installed and never saw this problem.

If rebasing it is complex for you I can just apply it manually, but then I'll have to drop this PR.

[set-soft]

Hi @telagod !
Do you still interested on it?