fix: clean up b2t2 slides (string_eq, unused vars, prettify)

Closes #2057

Operates on every slide under src/web/init/docs/ and src/b2t2/slides/
using the slide-* CLI subcommands:

1. Replaces the removed `string_eq(a, b)` with polymorphic `a == b` in
   the 9 slides that still referenced it (Tables docs, Aggregate,
   Ordering, AccessSubcomponents, Subtable, DataCleaning, MissingValues,
   groupByRetentive, groupBySubtractive). Manual refractors (the probes
   and statics annotations on Tables.ml in particular) are preserved by
   round-tripping ^^probe(...) / ^^statics(...) through the parser's
   trigger syntax — refractors stay populated after the rewrite.

2. Prefixes every pattern-binding flagged by statics as an unused
   variable with an underscore, silencing the warning (`requires`,
   `ensures`, destructured pattern vars like `score`, `l`, `v`, etc.).

3. Pretty-prints all slides at width 140. The wider width keeps every
   row of the multi-column data tables (sightings in Tables.ml, the
   jellyAnon table in pHackingHomogeneous/Heterogeneous) on a single
   line — width 80 broke the last row across many lines.

Modules.ml is intentionally left at origin/dev: PrettySegment.prettify
mishandles module syntax (each `;` separator over-indents the next
item), so prettifying it would make it strictly worse. That prettifier
bug is out of scope for this PR.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: 7h3kk1d