add vst3 ui snapshots

[erikgrahn13]

Summary

• add SNAPSHOT and SNAPSHOT_2X options to singularity_configure_vst3
• generate the processor-CID filenames required by VST3 hosts
• package the generated images with smtg_target_add_plugin_snapshots
• demonstrate the feature in ExampleEffect and document the public CMake API

Why

VST3 snapshot discovery requires filenames to start with the processor class ID. Singularity generates processor IDs internally, so this keeps that VST3-specific naming detail out of plugin projects while retaining a minimal opt-in API.

Implements the VST3 UI snapshots backlog item.

Validation

• Debug configure completed successfully
• ExampleEffect_VST3 Debug target built and packaged the CID-named snapshot
• Release VST3 validator discovered the snapshot and passed all 47 tests
• git diff --check passed

Tested on macOS.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Require the base snapshot before accepting the 2× variant

When a caller supplies only SNAPSHOT_2X, this condition treats the configuration as valid and packages only <cid>_snapshot_2.0x.png. The documented high-resolution image is an optional variant of SNAPSHOT; without <cid>_snapshot.png, hosts that request the standard-resolution filename will show no preview. Reject SNAPSHOT_2X unless SNAPSHOT is also provided.

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