ONNX search provider

[ollieatkinson]

Summary

Finishes the ONNX search spike into a narrow shippable shape for Linux/Android follow-up, with Windows left as the next runtime path.

• Adds model-owned ONNX configuration for tokenizer identity, pooling, normalization, input/output names, and query/document prefixes.
• Adds ONNX presets for MiniLM, BGE-small, GTE-small, and E5-small-v2.
• Moves query/document prefixing behind provider role methods, so the search core does not hardcode model instructions.
• Adds CLexiconONNXRuntime, a small C shim that dynamically loads ONNX Runtime for Linux/Android/Windows builds.
• Extends the SwiftPM setup plugin to fetch model artifacts plus Linux, Android, and Windows runtime artifacts.
• Keeps model comparison/evaluation outside the product CLI so the shipped surface remains small.
• Updates README and search platform docs with setup commands, platform notes, and cache identity.

Validation

Latest local validation after merging current trunk:

• swift build --product lexicon -Xswiftc -warnings-as-errors
• swift build --product lexicon --traits MLXSearch -Xswiftc -warnings-as-errors
• swift build --product lexicon --traits ONNXSearch -Xswiftc -warnings-as-errors
• swift build --product lexicon --traits MLXSearch,ONNXSearch -Xswiftc -warnings-as-errors
• swift test -Xswiftc -warnings-as-errors

Earlier spike validation also covered artifact setup:

• swift package --disable-sandbox --allow-writing-to-package-directory setup-onnx-search-artifacts -- --preset all-MiniLM-L6-v2
• swift package --disable-sandbox --allow-writing-to-package-directory setup-onnx-search-artifacts -- --skip-model --runtime linux-x64
• swift package --disable-sandbox --allow-writing-to-package-directory setup-onnx-search-artifacts -- --skip-model --runtime android
• swift package --disable-sandbox --allow-writing-to-package-directory setup-onnx-search-artifacts -- --preset bge-small-en-v1.5
• swift package --disable-sandbox --allow-writing-to-package-directory setup-onnx-search-artifacts -- --preset gte-small
• swift package --disable-sandbox --allow-writing-to-package-directory setup-onnx-search-artifacts -- --preset e5-small-v2

Cross-build attempts for Linux and Android reached the Lexicon ONNX/C shim sources, then stopped because the installed Swift SDKs were 6.3.1 while the host compiler was 6.3.2. The docs call out that the Swift SDK must match the host compiler version.