fix(export): emit drill files for every plated via layer span in gerbers export

[creationsunitassistant]

Summary

Fixes #4346

tsci export --format gerbers called convertSoupToExcellonDrillCommands once without layer_span, so the converter selected only the default F.Cu–B.Cu span. Every pcb_via whose span is not top-to-bottom was silently omitted from the drill files — the export exits successfully with no warning, producing fabrication files with open circuits.

Fix

Enumerate all distinct plated layer spans present in the circuit JSON (from pcb_via / pcb_plated_hole from_layer/to_layer) and emit one drill file per span:

• drill.drl — unchanged, default top-to-bottom span
• drill_top__inner1.drl, drill_inner1__bottom.drl, etc. — one per non-default span

Verification

• Reproduced the bug first: 4-layer board with 3 vias (top-bottom, top-inner1, inner1-bottom) exported only 1 drill hit; the two blind/buried vias were dropped.
• After fix: 3 drill hits across 3 span files.
• New regression test: tests/cli/export/export-gerbers-multi-span-drills.test.ts (passes)
• bun x tsc --noEmit -p tsconfig.json clean
• Existing export tests unaffected

Notes

• Uses the existing layer_span option already supported by circuit-json-to-gerber — no library changes needed.
• Fabricators treat multiple PTH files with distinct TF.FileFunction spans as standard (matches KiCad's per-span drill output).

🤖 This PR was prepared with AI assistance (Claude via Hermes agent). Happy to adjust.

[creationsunitassistant]

Hi! I reproduced this on a 4-layer fixture (3 vias: F.Cu–B.Cu, F.Cu–In1, In1–B.Cu — the same span mix from your repro table) and confirmed only the through vias made it into drill.drl. Fix emits one drill file per span using the converter's existing layer_span option.

If there's a bounty attached to this issue, happy to claim it properly — first PR to this repo, aiming to become a regular contributor.