feat: support arbitrary custom properties on schematic components

Promotes BOM / sourcing fields (MPN, Manufacturer, DigiKey_PN, LCSC,
JLCPCB_PN, Voltage, Tolerance, Dielectric, ...) to first-class citizens
on placed schematic symbols.

New MCP tools:
- set_schematic_component_property: add or update one custom property
  on a component (convenience wrapper around edit_schematic_component).
- remove_schematic_component_property: delete one custom property.
  The four built-in fields (Reference, Value, Footprint, Datasheet) are
  protected and rejected.

edit_schematic_component enhancements:
- New `properties` parameter: map of property name to either a string
  value or a full spec object { value, x?, y?, angle?, hide?, fontSize? }.
  Adds the property when missing, otherwise updates the existing field
  (and optionally its label position / visibility). Lets a single tool
  call attach an entire BOM payload to a component.
- New `removeProperties` parameter: list of custom property names to
  delete in the same call.
- Property values are now backslash-escaped so descriptions containing
  a double-quote or a backslash no longer corrupt the .kicad_sch file.
- New properties default to (hide yes) so they appear in BOM exports
  without cluttering the schematic canvas.

get_schematic_component description clarified to highlight that it
already returns every field on the symbol, including custom ones.

New MCP prompt component_sourcing_properties guides agents through the
conventional property names recognised by downstream BOM tooling and
the recommended call sequence.

Implementation (python/kicad_interface.py):
- _PROTECTED_PROPERTY_FIELDS frozenset
- _escape_sexpr_string / _find_matching_paren static helpers
- _set_property_in_block / _set_hide_on_property /
  _remove_property_from_block surgical text-level edits that preserve
  formatting and the property's UUID
- _handle_edit_schematic_component rewritten to orchestrate
  add/update/remove and return a per-property summary
- New handlers _handle_set_schematic_component_property and
  _handle_remove_schematic_component_property registered in the
  command dispatch table

Tests (tests/test_schematic_component_properties.py):
32 tests covering escape helper, paren matcher, add/update/remove
(single + batched), full spec dicts, default position, default
(hide yes), special-character escaping, UUID preservation, protected
built-in field rejection, no-op removal, both new convenience tools,
and input validation. All 590 tests in the project still pass.

Docs: README, SCHEMATIC_TOOLS_REFERENCE, TOOL_INVENTORY, CHANGELOG.

README.md

@@ -318,8 +318,10 @@ Complete schematic workflow with dynamic symbol loading (~10,000 symbols) and in
 
 - `add_schematic_component` - Place symbols from any KiCad library
 - `delete_schematic_component` - Remove component
-- `edit_schematic_component` - Edit properties and fields
-- `get_schematic_component` - Get component info with field positions
+- `edit_schematic_component` - Edit footprint, value, reference, label positions, and **arbitrary custom properties** (MPN, Manufacturer, DigiKey_PN, LCSC, Voltage, Tolerance, Dielectric, …) in one batched call
+- `set_schematic_component_property` - Add or update a single custom property (BOM/sourcing field) on a component
+- `remove_schematic_component_property` - Delete a single custom property from a component
+- `get_schematic_component` - Inspect every field on a component (built-in + custom) including label positions
 - `list_schematic_components` - List all components
 - `move_schematic_component` - Reposition component
 - `rotate_schematic_component` - Rotate component