Clean up b2t2 slides: string_eq, unused vars, prettify (#2057)

[7h3kk1d]

Summary

Closes #2057.

Operates on every slide under src/b2t2/slides/:

1. Replaces removed string_eq(a, b) with a == b in the slides that still referenced it (TableAPI Aggregate / Ordering / AccessSubcomponents / Subtable / DataCleaning / MissingValues, ExamplePrograms groupByRetentive / groupBySubtractive).

2. Prefixes pattern-binding tiles flagged as unused variables with _ to silence those warnings. Covers requires, ensures, destructured pattern names like score, l, v, etc. across every slide.

3. Pretty-prints all slides at width 140. Width 140 keeps every row of the multi-column data tables on a single line — at width 80 the last row of the jellyAnon table (pHacking{Homo,Hetero}geneous.ml) splits across many lines.

The src/web/init/docs/ slides are intentionally excluded from this PR — those have more custom formatting that the automated regeneration would disturb, so they'll be cleaned up separately.

The slide rewrites were generated with the slide-* CLI subcommands in #2282, but the resulting .ml files stand on their own — this PR only contains the regenerated slide content.

Test plan

• make dev builds cleanly
• B2T2 / Table API slides (Aggregate, Ordering, AccessSubcomponents, Subtable, DataCleaning, MissingValues) load cleanly with == replacing string_eq
• B2T2 / Example Programs (groupByRetentive, groupBySubtractive) load cleanly
• Pedagogical-error slides (Errors/*) still demonstrate their intended demo errors
• jellyAnon tables (pHacking{Homo,Hetero}geneous) — every row fits on a single line in the editor

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.40%. Comparing base (82b4fb5) to head (f0e0b58).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #2283      +/-   ##
==========================================
- Coverage   50.42%   50.40%   -0.03%     
==========================================
  Files         296      296              
  Lines       39453    39453              
==========================================
- Hits        19896    19888       -8     
- Misses      19557    19565       +8     

see 11 files with indirect coverage changes

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