Fix: dedupe model/quantization types, buffer downloads, GGUF offsets and device scanner isolation

[NightVibes33]

Motivation

• Resolve duplicate type declarations and conflicting QuantizationError enums that caused build errors and runtime mismatches.
• Prevent catastrophic OOM/slow downloads by replacing per-byte writes with buffered writes and ensure destination files exist before opening FileHandle.
• Make GGUF building and quantization safer and correct (stable tensor offsets, correct block-size math, correct Q4 nibble encoding) and avoid silently incorrect 5-bit fallbacks.
• Harden device detection and scanning concurrency to avoid unsafe @unchecked Sendable usage and potential reentrancy issues.

Description

• Centralized model/domain types and errors into ModelTypes.swift and removed duplicate declarations from ModelQuantizer.swift, keeping ModelQuantizer focused on orchestration.
• Buffered streamed downloads in ModelQuantizer.downloadModel and HuggingFaceAPI.downloadModelFile with a 64KB buffer and ensured destination files are created before opening FileHandle.
• Fixed Hugging Face fallback to use the API file-tree endpoint (/api/models/{id}/tree/main) and added a private getAuthToken() accessor into the HF API extension.
• Stabilized GGUFBuilder offsets by writing tensor-info into a temporary buffer before emitting tensor data and removed duplicate integer-to-data extensions.
• Improved QuantizationEngine behavior by inferring architecture from config.json model_type, clamping/offsetting Q4 quantized nibbles into [-8,7] + 8, computing block sizes with ceil (i.e. (numElements+31)/32 * elementSize), and changing q5_0/q5_1 to explicitly error until correctly implemented.
• Hardened DeviceScanner by making it @MainActor final (removed @unchecked Sendable), replacing Timer with a cancellable Task loop, adding deviceIdentifier to the profile, and adding simulator passthrough for model identifier parsing.
• Replaced iphone.gen3 SF symbol usage with the safer iphone fallback in views for broader compatibility.

Testing

• Ran git diff --check to verify whitespace/formatting issues and it returned no problems.
• Confirmed swift --version is available in this environment but an iOS build could not be executed because xcodebuild is not present (xcodebuild: command not found).
• No automated unit/integration tests were present or run in this environment, so functional build and runtime verification should be performed in an Xcode environment (iOS device/simulator) before release.

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Codex Task