fix: change snap_to_grid default grid from 2.54mm to 1.27mm (50 mil)

Half of all valid KiCAD schematic pin positions are on the 50-mil
(1.27mm) grid but not the 100-mil (2.54mm) grid — e.g. 26.67mm = 21 ×
1.27mm. Snapping to 2.54mm displaced those coordinates by 1.27mm,
moving labels off their pins and increasing floating-label count.

KiCAD source confirms: DEFAULT_CONNECTION_GRID_MILS = 50 and the ERC
off-grid check uses exact integer modulo against this value, so any
displacement breaks connectivity unconditionally.

Also update the kicad-source absolute path in CLAUDE.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -0,0 +1,60 @@
+# KiCAD MCP Server
+
+## Related Source
+
+- KiCad source code is located at `../kicad-source/` (absolute: `/home/eugene/Projects/kicad-source/`)
+
+## Testing
+
+### When to Write Tests
+
+Write tests for every non-trivial change to Python handler or business logic code:
+
+- **New MCP tools** — add tests for schema validation, handler dispatch, parameter validation, and the core logic path (happy path + key error cases).
+- **Changes to existing tools** — add tests covering the changed behaviour; update any tests that no longer reflect reality.
+- **Bug fixes** — add a regression test that would have caught the bug before adding the fix.
+- **Refactors that delete or rename public methods** — add a test asserting the old name no longer exists (see `TestConnectionManagerOrphanedMethodsRemoved` for the pattern).
+
+You do **not** need tests for TypeScript/TS-layer glue code that only forwards calls to Python (the TS test runner is not yet configured).
+
+### Test Levels
+
+| Level       | Use for                                                                                            | Marker                     |
+| ----------- | -------------------------------------------------------------------------------------------------- | -------------------------- |
+| Unit        | Schema shape, parameter validation, pure logic, mock-heavy handler dispatch                        | `@pytest.mark.unit`        |
+| Integration | Real file I/O against a `.kicad_sch` / `.kicad_pcb` copy; WireManager, JunctionManager round-trips | `@pytest.mark.integration` |
+
+Keep unit tests free of file I/O. Keep integration tests free of business-logic assertions that belong in unit tests.
+
+### Where to Put Tests
+
+```
+tests/
+  test_<feature_name>.py       # all Python tests go here
+```
+
+Group related test classes inside a single file (e.g. `TestSchemas`, `TestHandlerDispatch`, `TestHandleAddSchematicWireRouting` all in `test_wire_junction_changes.py`). Name classes `Test<Area>` and methods `test_<what_is_verified>`.
+
+Use `python/templates/empty.kicad_sch` as the base fixture for integration tests — copy it to a `tempfile` directory, run the handler, then parse the result with `sexpdata`.
+
+### Running Tests
+
+Always use the `.venv` virtualenv for Python commands:
+
+```bash
+npm run test:py                 # pytest tests/ -v
+.venv/bin/pytest tests/ -v      # all Python tests
+.venv/bin/pytest -m unit        # unit tests only
+.venv/bin/pytest -m integration # integration tests only
+.venv/bin/pytest --cov=python   # with coverage report
+.venv/bin/mypy python/          # type checking
+```
+
+## Git Workflow
+
+- **Never open a pull request automatically.** Commit and push when asked, but always wait for explicit instructions before running `gh pr create` or any equivalent command.
+
+## Python Code Style
+
+- **Never use `assert` in production code** — raise a specific exception (`ValueError`, `RuntimeError`, etc.) instead. `assert` is stripped in optimised builds and gives poor error messages.
+- **Do not introduce logic-breaking workarounds to satisfy the type checker** (e.g. `x or ""` when `""` is not a valid substitute for `None`). Fix the types or narrow with a proper guard (`if x is None: raise ...`).