Expose section plane in render_preview MCP tool

[w1ne]

Closes #514

Problem

headlessRender already supports a single axis-aligned clip plane (it's on the kernelcad render --section CLI), but render_preview never surfaced it. The agent could only ever see a model's outer shell — never verify wall thickness, internal pockets, or whether a bore runs through.

Change

• New render_preview input: { section: { axis: 'x'|'y'|'z', position: number, flip?: boolean } }.
• Reuses the render CLI's parseSectionFlag so positionRaw is kept verbatim — stringifying the number would emit exponent notation the page-side ?section= regex silently rejects, producing an unclipped render.
• Validates the plane and refuses an invalid one instead of rendering a misleading uncut view.
• Forwards { axis, position, positionRaw, flip } straight to headlessRender, beside the existing environment pass-through.
• Schema + tool description updated so the agent knows it can cut a cross-section to inspect interior geometry.

Tests (TDD)

New renderPreview.section.test.ts (4 cases): forwards a validated plane with verbatim positionRaw; preserves negative/decimal positions and flip; omits section when absent; refuses a non-finite position. MCP toolNameConsistency and toolRegistry.publicContract gates pass.

Scope

Agent-facing render param only. Out of scope: multi-plane corner cutaway, a Studio UI section panel, and a sectionView() script API.