WIP: Improve FIT file parsing with rust

[edance]

Changes

• FIT parsing happening in rust instead of in elixir for faster parsing
• Need to figure out how to run tests - Cant run `cargo test` on linux rusterlium/rustler#662
  Right now it needs all erlang/BEAM to test it out

Testing

1. Install dependencies:

cd native/rust_fit
cargo build

2. Launch IEx:

iex -S mix

3. Test with a sample FIT file:

activity = Squeeze.RustFit.parse_fit_file("/path/to/marathon.fit")

# Verify new fields exist
activity.trackpoints |> Enum.take(1) |> IO.inspect()

# Check lap timestamps
activity.laps |> Enum.map(&(&1.start_date_local)) |> IO.inspect()

# Verify activity start time
IO.puts activity.start_at_local

TODO

• Add running dynamics fields (ground contact time, vertical oscillation, etc)
• Fix start_date_local for laps
• Fix start_date_local for activity
• Add comprehensive tests
• Further code refactoring for maintainability

Next Steps

1. Add timezone conversion logic
2. Write unit tests for new fields
3. Add documentation for new fields
4. Consider adding more running metrics