Make formatter KiCad-style compliant and simplify stackup patching

[akhilles]

Previously, layout sync wasn't quite idempotent. It was only idempotent after KiCad loads + saves the layout file and fixes the formatting. Now, it's really idempotent. This also eliminates the need for complex change detection + conditional patching. The logic is now much simpler: always patch board config and then format.

---

Note

Medium Risk
Changes how .kicad_* files are rewritten during layout sync and introduces new formatting logic, so small formatting/ordering differences could create large diffs or expose parser/formatter edge cases despite added tests.

Overview
Makes KiCad S-expression output deterministic and KiCad-style compliant by introducing a new pcb_sexpr::formatter (prettify/format_tree) and switching schematic/symbol generation to use it.

Simplifies pcb layout stackup/layer patching by removing approximate-diff/conditional updates and instead structurally mutating the parsed PCB S-expression, then always rewriting the board with canonical formatting (atomic buffered write).

Extends pcb fmt to optionally format a single explicit KiCad S-expression file path (with --check/--diff support via unified diffs), while keeping default/discovery behavior limited to .zen files; updates snapshots accordingly and adds similar as a dependency.

^{Written by Cursor Bugbot for commit 6eeaaca. This will update automatically on new commits. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is OFF. To automatically fix reported issues with Cloud Agents, enable Autofix in the Cursor dashboard.}

[cursor]

Atomic rewrite fails on existing Windows files

Medium Severity

write_text_atomic_buffered uses fs::rename(&tmp_path, path) to replace an existing PCB file. On Windows, rename does not overwrite existing destinations, so stackup patching can fail even after successful write of the temp file.