Add placevr capture model with template validation, tests, and demo main

[EthanForAi]

Motivation

• Implement a fixed 8-shot capture model for the MVP to standardize guided captures and upload readiness.
• Provide a minimal Node/Shot data model and validation to enforce UX constraints and predictable uploads.
• Add a small demo in main.go to build a sample node, populate shots, validate, and emit JSON for integration testing.

Description

• Add new placevr package with capture.go defining MaxShotsPerNode, CaptureTemplate, GuideSlot, Shot, and Node types and validation logic including DefaultTemplateV1, CaptureTemplate.Validate, Node.Validate, Node.AddShot, and Node.IsReadyForUpload.
• Add unit tests in placevr/capture_test.go covering the default template validation and preventing adding more than MaxShotsPerNode shots.
• Update main.go to create a demo Node, populate it with shots up to MaxShotsPerNode, validate it, marshal it to JSON, and print upload readiness.
• Add go.mod with module placevr and go 1.22.

Testing

• Ran unit tests with go test ./..., and the tests completed successfully.

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Codex Task