fix: avoid injecting unsupported --variant flag into kicad-cli

[ril3y]

Summary

_active_variant_args() was injecting --variant default into every
kicad-cli invocation when no variant override was active, because the
default sidecar state ships with active_variant: "default". KiCad CLI
9.x doesn't recognize --variant and aborts with Unknown argument: --variant. The error went to stdout, not stderr, so the wrapper's
stderr or 'unknown error' collapsed to unknown error and masked the
root cause.

PCB and schematic exports failed in lockstep against KiCad 9.0.7 on
Windows. Reproducible with any 9.x kicad-cli.

Changes

• discovery.py — add supports_cli_variant capability flag, populated
  from "--variant" in <kicad-cli help blob>. KiCad 10 introduced the flag.
• tools/export.py:_active_variant_args — suppress the synthetic no-op
  --variant default unconditionally; for explicit non-default variants,
  gate emission on caps.supports_cli_variant.
• tools/export.py:_export_sch_pdf — fall back to stdout when stderr is
  empty so future arg-parse errors surface in the user-visible message.
• Tests — set supports_cli_variant=True on the mock cap in
  test_export_tools.py and test_release_hardening.py so the existing
  --variant lite forwarding assertions continue to exercise the
  KiCad-10 path.

Test plan

• pytest -k export — 34 passing
• Full suite + coverage gate (npm run check) — 90.15% coverage
• Repro: export_sch_pdf against a real project on KiCad 9.0.7 —
  fails on main, succeeds on this branch (PDF written, 1.78 MB)
• No behavior change on KiCad 10 CLIs — variant flag still forwarded
  when capability probe sees it

Notes on local CI

Three integration tests fail on KiCad 9.0.7 environments regardless of
this branch:

• test_kicad_cli_smoke.py::test_kicad_cli_version_is_discoverable
  asserts KICAD_MAJOR_VERSION (10).
• test_kicad_cli_smoke.py::test_kicad_cli_exports_board_stats_without_gui
  uses pcb export stats, a KiCad-10-only subcommand.
• test_pcb_tools.py::test_pcb_design_blocks_and_inner_layer_graphics_success
  fails on a stackup-validation drift (asserts inner-layer add succeeds,
  but the implementation now requires >=4 copper layers; the fixture
  has 3).

All three fail identically on main in the same environment. They are
unrelated to this change. They should pass on a KiCad-10 CI environment.

[gemini-code-assist]

Code Review

This pull request introduces support for the --variant flag in the KiCad CLI, which is available in KiCad 10+. It includes capability detection for this flag and updates the export logic to use it when appropriate, while suppressing it for the 'default' variant. Error reporting for schematic PDF exports is also improved by including standard output in failure messages. Feedback suggests raising a ValueError instead of silently ignoring requested variants when the CLI lacks support, ensuring users are aware of compatibility issues that could lead to incorrect exports.

[gemini-code-assist]

Silently returning an empty list when a non-default variant is requested but unsupported by the CLI can lead to serious correctness issues. If a user (or the sidecar state) has specified a variant like lite, and the system silently exports the default (full) design because the CLI is too old, the user may inadvertently manufacture incorrect boards.

Since _active_variant_args already raises ValueError for invalid variant names, it should also raise a ValueError here to explicitly inform the user that their requested variant cannot be applied due to CLI version constraints.

Suggested change

	if not caps.supports_cli_variant:
	return []
	if not caps.supports_cli_variant:
	raise ValueError(
	f"The detected KiCad CLI does not support the '--variant' flag. "
	f"Cannot apply variant '{args[1]}'."
	)

[oaslananka]

Please raise ValueError when caps.supports_cli_variant is False and a non-default variant is active, as suggested by Gemini. Silent suppression risks exporting wrong board variant.

[oaslananka]

Thanks for the KiCad 9 kicad-cli --variant fix.

This repository is now a personal showcase mirror; the canonical repository is:

https://github.com/oaslananka-lab/kicad-mcp-pro

I ported this fix into the canonical org repo and merged it as:

oaslananka-lab/kicad-mcp-pro#12

Canonical merge commit:

b2b7537

The fix keeps --variant default from being injected into KiCad 9.x CLI calls, gates explicit non-default variants on detected CLI support, and preserves KiCad 10 variant behavior.

Closing this PR here to avoid personal-repo divergence. Future changes should target:

https://github.com/oaslananka-lab/kicad-mcp-pro