Visualize ROS2 IMU covariance diagnostics

[RitwijParmar]

What

Adds covariance handling to the ROS2 sensor_msgs/msg/Imu MCAP parser.

• preserves the raw 3x3 orientation, angular-velocity, and linear-acceleration covariance matrices as queryable custom components on the IMU entity
• emits chartable per-axis standard-deviation time series under covariance/*_stddev child entities
• treats ROS unknown/missing covariance sentinels correctly by logging NaNs for the derived stddev series instead of misleading confident zeros

This is intended as a scoped first step toward #10728 without adding a new viewer primitive or broader covariance rendering feature.

Why

Robotics users inspecting ROS2 MCAP recordings often need to debug estimator uncertainty alongside gyro/accelerometer values. Today the IMU parser drops covariance matrices entirely, so users need custom preprocessing before they can see whether an IMU stream is reporting known, missing, or changing uncertainty.

Tests

• cargo test -p re_mcap covariance_stddevs --no-default-features
• cargo test -p re_mcap --no-default-features ran 42/43 tests locally. The only failure was decoders::attachments::tests::test_attachments_mcap_fixture with failed to read summary: Bad magic number, which appears unrelated to this patch and tied to the local binary fixture in this shallow clone.

[github-actions]

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[RitwijParmar]

Looks like the only remaining red check is the changelog label. I do not have label permissions here. My read is that this can be included in the changelog since it adds ROS2 IMU covariance diagnostics, but happy with whichever label maintainers prefer.