[Claude-led] Feature/spec collate

[dougbrn]

Change Description

First implementation PR for the initial design (docs/design/initial-design.md, §4).

Solution Description

• Adds lsdb>=0.11 and torch>=2.4 as core dependencies.
• ColumnSpec / Flat / Nested: maps flat and nested catalog columns to tensors. resolve() validates against catalog meta (nested sub-column types via NestedDtype.column_dtypes) without computing any data, and reports every problem in one error.
• Nested columns are padded to the batch maximum with a <key>_mask and <key>_lengths, fully vectorized with nest.len() / nest.to_flat(). Rows with empty or null nested values have length 0. Options: max_length with head/tail truncation, pad_to, pad_value, null fill, transform=.
• resolve_device / move_batch helpers (default "cpu", "auto" opt-in).

Tests compare against a per-row reference implementation and include a fixture catalog that raises if any partition is computed.

Note: CI only triggers on main, so this PR gets no CI until dev is added to the workflow triggers (design doc §12).

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 95.50173% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.59%. Comparing base (c9ed114) to head (e266875).

Files with missing lines	Patch %	Lines
src/lsdb_ml/spec.py	95.79%	9 Missing ⚠️
src/lsdb_ml/device.py	86.36%	3 Missing ⚠️
src/lsdb_ml/collate.py	98.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##               dev       #6      +/-   ##
===========================================
- Coverage   100.00%   95.59%   -4.41%     
===========================================
  Files            2        5       +3     
  Lines            7      295     +288     
===========================================
+ Hits             7      282     +275     
- Misses           0       13      +13     

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