type: add type annotations to 1D solvers

[formeo]

Related to #45

What changed

All 5 files in dimension_1/solvers/ now have full type annotations:

• Function signatures: parameter types + return types
• Local variables: annotated where non-obvious
• NumPy docstrings: updated with types in Parameters/Returns sections
• TYPE_CHECKING guard used for Object import (avoids circular import)

Runtime type checks added to all public functions:

• apply_latent_heat() raises TypeError if nx is not a list
  or obj lacks required thermal attributes
• All 4 solver functions raise TypeError if obj is not a thermal
  Object (checked via hasattr(obj, 'num_points'))

New test file: test/unit/dimansion_1/test_solvers_1d.py

• 14 tests covering TypeError for all solvers + apply_latent_heat
• Parametrized across all 4 solvers (string, None, int inputs)

What didn't change

• Solver logic, FDM stencils, latent heat computation
• Function signatures (no new parameters)
• Existing test results

Testing

All 40 tests pass (26 existing + 14 new).
Solver coverage: 97% (explicit/implicit), 68% (_latent_heat).

[codecov]

Codecov Report

❌ Patch coverage is 95.18072% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.06%. Comparing base (71522f1) to head (4c585e2).

Files with missing lines	Patch %	Lines
heatrapy/dimension_1/solvers/_latent_heat.py	69.23%	4 Missing ⚠️

@@             Coverage Diff             @@
##           develop      #75      +/-   ##
===========================================
+ Coverage    72.79%   73.06%   +0.26%     
===========================================
  Files           22       22              
  Lines         2272     2294      +22     
===========================================
+ Hits          1654     1676      +22     
  Misses         618      618              

Files with missing lines	Coverage Δ
heatrapy/dimension_1/solvers/explicit_general.py	96.55% <100.00%> (+0.55%)	⬆️
heatrapy/dimension_1/solvers/explicit_k.py	96.55% <100.00%> (+0.55%)	⬆️
heatrapy/dimension_1/solvers/implicit_general.py	96.66% <100.00%> (+0.51%)	⬆️
heatrapy/dimension_1/solvers/implicit_k.py	96.55% <100.00%> (+0.55%)	⬆️
heatrapy/dimension_1/solvers/_latent_heat.py	67.74% <69.23%> (+7.74%)	⬆️

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