cleanup: strip dead motif/cys PFrame setup from specs.lib.tengo

The motifs and cysteines per-residue PFrames are out of scope in the
refreshed spec ("Out of scope: per-residue liability drill-down"), so
the column-spec dictionaries and Xsv builders that produced them have
been dead since the spec refresh. main.tpl.tengo only consumes
`specs.buildScoresXsvSettings`; everything else was orphaned.

Dropped:
- `motifAxesSpec`, `motifColumnSpecs`, `motifColumnIds` (~100 lines)
- `cysAxesSpec`, `cysColumnSpecs`, `cysColumnIds` (~120 lines)
- `motifAxisParquetMapping`, `cysAxisParquetMapping`
- `buildMotifsXsvSettings`, `buildCysXsvSettings`
- `xsvAxesFromMapping` (only the dead builders used it)
- `scoresAxesSpec`: unused single-element placeholder from the
  `pt.saveFrameDirect` era. The single-shot `xsv.importFile` path
  takes the scClonotypeAxisSpec as an argument; no placeholder needed.

Folded `buildXsvColumns(columnIds, columnSpecs, blockId)` to take just
`blockId` (the only scores frame uses the module-level scoresColumnIds
and scoresColumnSpecs directly).

Header comment rewritten to describe the actual current state: scores
PFrame only, single-shot Python pipeline, motif/cys out-of-scope.

Net: 671 to 337 lines (half the file gone). Block build 7/7; the only
consumer (`main.tpl.tengo`) still imports `specs.buildScoresXsvSettings`.

Author: xnacly