PLN: "so many tracks still having ^42 instead of ^41 for kicks its confusing."

Remap phase 2 (5910aacc) column-aligned the board so column N IS orbit N, but it
was scoped to the FOURTEEN files of the OPAL set. Everything else still spoke
the layout from when gMask held CC 41, where an orbit's gate row started one
column late: d1 -> ^42, d2 -> ^43. button-column-audit found 169 such files,
every one of them shift +1 — a single layout generation, not a scatter of typos.

Layout-shifted files: 169 -> 0. Buttons on their own column: 35.7% -> 78.6%.

TWO SYSTEMIC FIXES IN migrate-columns.py, both found by running it on the whole
corpus instead of a setlist. Each refused a batch rather than writing bad
Haskell, which is the only reason they were cheap to find:

1. THE SEGMENT BOUNDARY WAS DEPTH-BLIND. A chain step ends at the next line
   starting with `$`/`#`, but a `$` inside an open paren is a CONTINUATION, not
   a new step. So a multi-line body truncated after one line, left depth 1, and
   the balance check refused the file. 10 of 169 were unmigratable. The boundary
   is now paren-aware, and the hit line is no longer assumed to BE a step start:
   `^NN` is routinely referenced from the middle of a body, so we walk back to
   the nearest enclosing step whose segment both balances and contains the hit.
   Negative depth was the tell that the anchor was too late, not that the source
   was broken.

2. COMMENTING OUT A `$`-STEP CAN UNDER-APPLY THE STEP ABOVE IT. Not a paren
   problem — an ARITY problem, and silent-eval is what caught it: 4 files
   stopped compiling while 165 were fine.

       $ midiOff "^57" (mask "<[t f]!0 ...>")   <- a function, still hungry
       $ stack [ ... ]                          <- its argument

   Comment the `stack` and GHC says "midiOff is applied to too few arguments"
   and the whole do-block dies, all twelve orbits — far worse than the
   cross-orbit trigger the comment-out was avoiding. `#`-steps stay
   commentable (`#` takes a ControlPattern, so a shorter chain is still
   well-typed); `$`-steps are now ANNOTATED and left live, and the FIXME says
   so, because an annotated line IS still firing.

VERIFIED against a pre-migration baseline, not against hope:
  * non-compiling tracks: 17 before, 17 after, and the SETS ARE IDENTICAL —
    zero newly broken, zero newly fixed. All 17 predate this work.
  * silent orbits: the same 19, in the same tracks, only line numbers moved.
  * pvlint: 30 -> 26 errors, no new error signature. Warnings 996 -> 1041: the
    +45 are the annotated-but-live `$`-steps honestly reporting that they still
    reach another column. Errors down, honesty up.
  * fix-button-roles: 3 clean role rewrites applied, then 0 remaining. The
    role report is otherwise unchanged from HEAD, so the migration introduced
    no gate/gesture inversion — the class only PLN's ear catches.
  * migrate-columns --plan re-run: 0 moves. Converged.

Left for a second pass, deliberately: 118 sites where an orbit's two own-column
buttons carry the same role (e.g. two gestures, no gate). Those are not
inversions — with no gate there is nothing to swap, and the grid gives d1-d3 one
button anyway. The count rose from 74 because column-aligning the buttons is
what made them VISIBLE; they were unevaluable before. gSel (#54) is the fix.