pcb layout stackup sync now also patches general (thickness ...)

[akhilles]

Also, only rewrite the subset of the layout file we're patching. Not the whole thing. We were already using this strategy for net rename and other Rust-based kicad file patching. We should use it for board config patching as well.

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Note

Medium Risk
Touches layout file patching/formatting logic and changes how .kicad_pcb content is rewritten; errors could cause malformed boards or noisy diffs, though changes are localized and covered by new tests.

Overview
pcb layout stackup sync now also patches general (thickness ...) using a computed board thickness (including KiCad’s fixed solder mask contribution) and formats the value in KiCad-style mm text.

Stackup/layers syncing is refactored to generate a PatchSet that replaces only the relevant S-expression spans (layers, setup.stackup, and general.thickness) and then runs prettify, reducing unrelated churn compared to reformatting the entire parsed tree.

pcb-sexpr parsing/formatting is extended to preserve original numeric lexemes (e.g. 12.000000) via a new raw_atom field, ensuring round-trips and targeted patches don’t normalize unrelated numeric precision.

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