fix: refactor public API to use interfaces instead of type aliases

[kolkov]

Summary

Refactors the public API packages to use proper Go interfaces instead of type aliases where possible. This improves pkg.go.dev documentation by hiding internal implementation paths and provides proper interfaces for external consumers.

Changes

• tensor/: Add Backend interface with all 40+ methods, use public tensor types
• nn/: Add Module interface with full method definitions, keep Parameter as type alias (required for Go's type system)
• onnx/: Add Model interface for ONNX model operations
• optim/: Use public nn.Parameter in function signatures
• autodiff/: Use public tensor types
• loader/: Keep ModelReader as type alias (internal types in method signatures)
• backend/cpu, backend/webgpu: Add compile-time interface checks

Technical Notes

Some interfaces (Optimizer, ModelReader) remain as type aliases because their method signatures reference internal tensor types. Go's type system treats internal/tensor.Backend and tensor.Backend as different types even when they have identical methods.

Test plan

• All existing tests pass (go test ./...)
• Linter passes with 0 issues (golangci-lint run ./...)
• Build succeeds for all packages
• New tests added for interface implementations

Fixes #25

[codecov]

Codecov Report

❌ Patch coverage is 10.81081% with 33 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
nn/module.go	0.00%	22 Missing ⚠️
onnx/onnx.go	0.00%	10 Missing ⚠️
backend/cpu/cpu.go	0.00%	1 Missing ⚠️

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