Restore ResampledTSDF intermediate object to prevent invalid post-resample operations

[R7L208]

Changes

Introduces a ResampledTSDF composition-based wrapper (like Spark's GroupedData) that only exposes valid post-resample operations (interpolate, as_tsdf, show). This prevents chained operations like .filter() after .resample() from silently invalidating resample context while metadata travels forward to .interpolate(), producing wrong results.

Key changes:

• New ResampledTSDF class (tempo/resampled.py): read-only wrapper with interpolate(), as_tsdf(), show(), and __repr__
• TSDF.__init__/__withTransformedDF: removed mutable resample_freq/resample_func attributes
• TSDF.resample() and resample.resample(): now return ResampledTSDF instead of TSDF
• TSDF.interpolate(): simplified to delegate through self.resample(...).interpolate(...)
• calc_bars() in both tsdf.py and stats.py: calls .as_tsdf() after each resample()
• __init__.py: exports ResampledTSDF

Linked issues

Resolves #458

Functionality

• added a new Class method
• modified existing Class method: TSDF.__init__, TSDF.__withTransformedDF, TSDF.resample, TSDF.interpolate, TSDF.calc_bars
• modified existing function: resample.resample, stats.calc_bars
• added a new test
• added a new utility

Tests

• manually tested
• added unit tests
  • test_resample_returns_resampled_tsdf — verifies return type and as_tsdf() escape hatch
  • test_tsdf_resample_returns_resampled_tsdf — verifies TSDF.resample() return type
  • test_resampled_tsdf_blocks_invalid_operations — verifies AttributeError on .filter(), .withColumn(), etc.
  • test_resampled_tsdf_repr — verifies descriptive __repr__
  • test_resample_then_interpolate_chain — verifies tsdf.resample(freq, func).interpolate(method) works end-to-end

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.55%. Comparing base (f022563) to head (93955c3).
⚠️ Report is 152 commits behind head on v0.2-integration.

Additional details and impacted files

@@                 Coverage Diff                  @@
##           v0.2-integration     #461      +/-   ##
====================================================
- Coverage             85.46%   78.55%   -6.92%     
====================================================
  Files                     6       31      +25     
  Lines                   757     2840    +2083     
  Branches                156      469     +313     
====================================================
+ Hits                    647     2231    +1584     
- Misses                   92      450     +358     
- Partials                 18      159     +141     

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