Silhouette: add restore/snapshot mechanism for round-trip toggle

## Summary

\`VRMRenderer.applySilhouetteMode(model:config:)\` is a one-way mutation: it sets renderer flags, reconfigures lights, and rewrites every material's albedo/emissive/outline state. There is no counterpart \`restoreFromSilhouette(model:)\` and the doc comment on \`applySilhouetteMode\` notes the limitation:

> Idempotent — safe to call before or after \`loadModel\`. Any subsequent gameplay-style scene reset would need to undo each step manually.

For host apps that want to toggle silhouette mode on/off in the same renderer instance (menu host transitions, accent shots), this is user-hostile — they have to reload the model.

## Suggested approach

Two parts, can ship together or separately:

**1. Renderer-side restore (cheap):**
- Snapshot the four renderer flags affected (\`disableAutoMaterialOverrides\`, \`additiveDirectionalRimEnabled\`, \`additiveDirectionalRimPower\`, plus the three light slots and ambient color) before mutating in \`applySilhouetteMode\`.
- Store the snapshot in a private \`silhouetteSnapshot\` property.
- Add \`restoreFromSilhouette()\` that reverts those.

**2. Material-side restore (heavier):**
- Snapshot per-material \`baseColorFactor\`, \`baseColorTexture\`, \`emissiveTexture\`, \`emissiveFactor\`, plus the MToon fields zeroed by silhouette (\`shadeColorFactor\`, \`matcapFactor\`, \`parametricRimColorFactor\`, \`giIntensityFactor\`, the four outline fields).
- Memory: ~80 bytes/material × ~15 materials = ~1.2 KB per model. Trivial.
- Reset via deep copy back into \`model.materials[i]\`.

Alternative: have the host maintain its own snapshot before calling \`applySilhouetteMode\` and restore via a fresh model load. This puts the burden on the caller but avoids API surface area inside the renderer.

## Discovered during

PR #137 review. Added unit-test coverage for \`applySilhouetteMode\` mutation invariants in \`8c2de51\`; a future restore mechanism would need the same coverage symmetrically (round-trip identity).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Start by reading VRMRenderer.applySilhouetteMode(model:config:) and the unit-test coverage added in commit 8c2de51 for its mutation invariants. Define the snapshot boundary between renderer and material state, then add symmetric round-trip tests showing that restoring after silhouette mode returns the model and renderer to their original state.

Written by the indexing model from the issue text.