fix: write every sheet file for hierarchical KiCad schematic exports

[wroscoe]

Problem

When a design uses schSheetName (hierarchical schematic), tsci export -f kicad_zip and tsci build --kicad-project / --kicad-project-zip produce a KiCad project whose schematic is empty:

• CircuitJsonToKicadSchConverter.getOutputString() returns only the root sheet for hierarchical schematics (this.files[0].content).
• The root sheet contains (sheet ...) blocks whose Sheetfile properties point at sheet_1.kicad_sch, sheet_2.kicad_sch, ...
• Those child files are never written, so KiCad opens an empty root sheet with broken sheet links, and every symbol is missing.

The export still exits 0 and the zip contains exactly one .kicad_sch, so nothing flags it. PCB output is unaffected.

We hit this on a 3-sheet board: the exported index.circuit.kicad_sch was 2.7 KB with 0 symbols and three Sheetfile references to files not in the zip.

Fix

circuit-json-to-kicad already provides getOutputFiles({ schematicFilename }), which returns the root plus every child sheet (and just the root for non-hierarchical designs). Use it in both places:

• lib/shared/export-snippet.ts: the kicad_zip export
• cli/build/generate-kicad-project.ts: build --kicad-project / --kicad-project-zip, which zips the output dir

GeneratedKicadProject.schContent still holds the root sheet, so its shape is unchanged. Output for non-hierarchical schematics is byte-for-byte the same.

Tests

• tests/cli/export/export-kicad.test.ts: a two-sheet circuit exported with kicad_zip. Asserts the zip contains the root plus sheet_1/sheet_2, that the root references both files, and that each child holds its symbol.
• tests/cli/build/build-kicad-project-zip.test.ts: the same circuit through build --kicad-project, asserting all three .kicad_sch files are written.

tsc --noEmit and biome format pass locally. I couldn't run the test suite locally because my sandbox blocks some outbound hosts the render path uses, so this PR is a draft until CI is green.

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