feat(cell_metrics): source RT-switching junction violins from the cell summary

[CarlosBlancoGo]

Summary

• Moves the two RT-switching-by-splice-junction-type violins (all junctions and
  unique junctions) out of the report's on-the-fly junctions×classification
  explosion and into real cell-summary columns, computed once during metrics —
  removing the last per-cell junction re-derivation from the report.
• Adds 12 per-cell columns in both modes, emitted only when the junctions
  file carries the needed inputs (so the report skips a plot exactly when its
  columns are absent — same behaviour as the old runtime guards).

Details

• cell_metrics.py
  • RTS_{Known,Novel}_{canonical,non_canonical}_junctions — RTS-flagged,
    FL-weighted numerators for the all-junction plot; denominators are the
    existing *_junctions totals.
  • unique_… + RTS_unique_… — distinct-intron counts for the unique-junction
    plot. Isoforms mode collapses the same genomic intron shared across
    transcripts via a sparse Jt @ Xp incidence (junction×transcript times
    transcript×cell presence), binarized and kept in scipy.sparse so the
    (junction, cell) product is never densified. Reads mode dedups introns per
    cell with a groupby.
  • Adds RTS_junction + genomic-coordinate columns to the otherwise-minimal
    junctions read.
• SQANTI-sc_report.R — new build_rts_sjtype_long() derives each SJ type's
  per-cell percentage from the columns (NA when a cell has no junctions of a
  type, so it drops out of the violin); both plots are gated on their columns'
  presence. Deletes the junc_rt explosion and the now-dead junc_iso_key
  resolver.
• Tests — 4 new (isoforms shared-intron-collapse and distinct-intron cases,
  reads-mode RTS/unique).