group emits null anchor_alignment and subcircuit_id, failing circuit-json validation

What happens

<group /> emits two fields as null where circuit-json accepts a string or nothing, so the elements fail validation:

<board width="30mm" height="30mm">
  <group name="G1" pcbX={4}>
    <resistor name="R1" resistance="1k" footprint="0402" />
  </group>
</board>

pcb_group:       anchor_alignment: Expected 'top_left' | ... , received null
schematic_group: subcircuit_id: Expected string, received null

Cause

Both are explicit nulls written at insert time:

// lib/components/primitive-components/Group/Group.ts:468
anchor_alignment: props.pcbAnchorAlignment ?? null,

// lib/components/primitive-components/Group/Group.ts:1590
subcircuit_id: this.subcircuit_id!,   // the field is `string | null = null`

The schemas are:

pcb_group.anchor_alignment  →  ninePointAnchor.default("center")
schematic_group.subcircuit_id  →  z.string().optional()

So undefined is correct in both cases — and for anchor_alignment it's strictly better than null, because omitting it lets the schema's own "center" default apply instead of storing a value the schema rejects.

A group placed outside any subcircuit legitimately has no subcircuit_id; the ! assertion just hid that it was null rather than a string.

These were already baked into expectations

Three existing tests record the invalid values, which is why nothing caught it:

• tests/groups/group-outline.test.tsx — "anchor_alignment": null
• tests/components/primitive-components/group-subcircuit-id.test.tsx — "anchor_alignment": null
• tests/groups/group-schematic-box.test.tsx — "subcircuit_id": null

Context

Third and final batch from validating core's output against circuit-json's own schemas (see #2841 for display_offset_x/y, #2843 for the hole elements). With this, <group /> no longer contributes schema violations.

PR ready.

Start in lib/components/primitive-components/Group/Group.ts at the two insertion sites identified in the issue, then review the expected values in tests/groups/group-outline.test.tsx, tests/components/primitive-components/group-subcircuit-id.test.tsx, and tests/groups/group-schematic-box.test.tsx. Run those tests and circuit-JSON validation; done means a bare group emits no rejected null fields and all expectations pass.