drive: static_assert that struct Log stays all-float

[eugenevinitsky]

Summary

`env_binding.h`'s `vec_log` iterates `struct Log` as a raw `float[]` via `sizeof(Log)/sizeof(float)`. The existing comment "Will break horribly if Log has non-float data" warns about it, but nothing enforces the invariant. Adding a `double` / pointer field or even an unintended `int` could silently type-pun across the aggregation path.

This PR ties a `LOG_NUM_FLOAT_FIELDS` macro to the field count and `_Static_assert`s at the struct boundary that `sizeof(struct Log) == LOG_NUM_FLOAT_FIELDS * sizeof(float)`. Adding a field requires bumping the count; forgetting to bump it, or sneaking in a wider-than-float field, becomes a compile error.

```c
#define LOG_NUM_FLOAT_FIELDS 54

struct Log {
float n;
...
float reward_ade;
};
_Static_assert(
sizeof(struct Log) == LOG_NUM_FLOAT_FIELDS * sizeof(float),
"struct Log size mismatch: a field was added without bumping "
"LOG_NUM_FLOAT_FIELDS, or a non-float field slipped in (would corrupt "
"vec_log's raw-float iteration).");
```

Gap

A same-sized non-float field (e.g. `int`) with the count bumped would still match `sizeof(Log) == N * sizeof(float)` and slip past the assert — the raw-float iteration would then type-pun the int bytes through `float`-typed reads. The comment at the top of the struct calls this out so a contributor is at least warned. A fully type-safe fix would need an X-macro field-registry refactor, which is much more invasive; this PR aims for the cheap, high-coverage defense.

Test plan

• `python setup.py build_ext --inplace --force` clean (only the pre-existing `maps_checked` warning).
• Verified that the assertion is meaningful: bumping the count to 55 in a sandbox triggered the expected compile-time error.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a compile-time guard to keep struct Log compatible with env_binding.h’s raw-float aggregation path.

Changes:

• Defines LOG_NUM_FLOAT_FIELDS next to struct Log.
• Adds documentation that every Log field must remain a float.
• Adds a _Static_assert ensuring sizeof(struct Log) matches the expected float field count.

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