Add per-timestep TFT variable importance plot

[exactml]

Checklist before merging this PR:

• Mentioned all issues that this PR fixes or addresses.
• Summarized the updates of this PR under Summary.
• Added an entry under Unreleased in the Changelog.

Addresses #2685.

Summary

Adds the plotting follow-up to #3170 (which added the per-timestep data layer but intentionally left visualization out of scope).

• New TFTExplainer.plot_variable_selection_over_time() plots the encoder/decoder variable importances from get_encoder_importance_over_time() / get_decoder_importance_over_time() as two line-chart subplots (one line per variable), instead of the stacked-bar approach originally prototyped in the issue thread.
• show_index_as: Literal["relative", "time"] = "relative" mirrors the existing plot_attention() convention. "relative" re-indexes both series around the first prediction point (0); "time" uses the real dates the importances were already computed against, unchanged.
• max_nr_components caps how many variable lines are drawn per subplot, reusing the same knob TimeSeries.plot() already exposes — addresses the "unreadable past ~20 features" issue @MichaelVerdegaal raised in the thread.
• Line plots (rather than stacked bars) were chosen specifically to avoid the dense/overlapping x-axis labels that stacked bars hit on long input_chunk_length series — verified TimeSeries.plot()'s existing date-axis handling already degrades gracefully on long date ranges.
• Left out of v1, per discussion: auto-merging cyclic sin/cos encoder pairs (a darts-encoder-specific naming heuristic, better left to the user), and "normalize by attention" (the original prototyper's own retrospective found it didn't pull its weight).

Other Information

Testing:

• Added test_variable_selection_over_time_plotting in test_tft_explainer.py, covering both n_series cases: subplot titles/count, per-variable line count, max_nr_components capping, and the show_index_as validation error.
• pytest darts/tests/explainability/test_tft_explainer.py — 54 passed.

[codecov]

Codecov Report

❌ Patch coverage is 96.77419% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.97%. Comparing base (080b534) to head (d8fbbf6).

Files with missing lines	Patch %	Lines
darts/explainability/tft_explainer.py	96.77%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3174      +/-   ##
==========================================
- Coverage   97.03%   96.97%   -0.06%     
==========================================
  Files         165      165              
  Lines       17951    17982      +31     
==========================================
+ Hits        17418    17438      +20     
- Misses        533      544      +11     

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[exactml]

@dennisbader can you review it when you available?