Add pcb export-kicad subcommand

[jeanmw]

Summary

Implements pcb export-kicad, a new CLI subcommand that takes a .zen
file and writes a standalone, self-contained KiCad project directory.

• New subcommand: pcb export-kicad <file.zen> -o <dir> [--offline] [--locked]
• --offline and --locked plumb through to the shared resolve::resolve(...)
  pipeline used by pcb build/pcb bom/etc.
• Refuses missing/non-.zen inputs with a clear error.
• Fixes a temp-dir leak in pcb_layout::process_layout(use_temp_dir=true):
  the TempDir is now owned by the returned LayoutResult and is removed
  when the result drops, instead of being kept via TempDir::keep().

Closes #682.

Notes for reviewers

• The exported directory contains layout.kicad_pro + layout.kicad_pcb
  (+ .kicad_prl). KiCad creates the .kicad_sch file itself the first
  time eeschema opens the project — same behavior as
  crates/pcb-layout/tests/layout_generation.rs.
• LayoutResult gained a private _temp_dir: Option<TempDir> field. All
  existing callers pass use_temp_dir=false so they get None and
  behavior is unchanged. Only the new export_kicad path passes true.

Test plan

• cargo test --workspace --locked --no-fail-fast
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check
• cargo run -p pcb -- fmt --check stdlib
• crates/pcb/tests/export_kicad.rs:
  • test_export_kicad_rejects_missing_file
  • test_export_kicad_writes_project_directory
  • test_export_kicad_offline_locked_writes_project_directory (hermetic,
    seeded sandbox cache, network egress blocked)
• Verified no pcb-layout-* directories leak into $TMPDIR after
  running the export tests.
• Manual smoke test: opened the exported layout.kicad_pro in KiCad
  and confirmed it loads cleanly.

---

Note

Medium Risk
Adds a new CLI command that generates and copies full KiCad projects, and changes process_layout(use_temp_dir=true) to manage temp directory cleanup via LayoutResult, which could affect workflows that relied on the previous temp dir persistence unless persist_temp_dir() is used.

Overview
Adds a new pcb export-kicad subcommand that builds a .zen design, runs process_layout in a fresh temp layout, then copies the generated KiCad project files into a user-specified output directory (with --offline/--locked wired through resolution) and reports diagnostics.

Fixes a temp-dir leak/behavior in pcb_layout::process_layout(use_temp_dir=true) by having LayoutResult own the backing TempDir and delete it on drop, plus introduces LayoutResult::persist_temp_dir() and updates pcb layout --temp to call it before launching KiCad asynchronously.

Adds tests and snapshots covering export-kicad success, missing input handling, and offline/locked execution, and updates the CLI help snapshot to include the new command.

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