Regenerate expander linklet from Racket v9.2

[pmatos]

Summary

• expander/expander.rktl had not been refreshed since the initial commit (2023). Regenerate it against Racket v9.2.
• Add scripts/gen-expander.sh, a self-contained script to reproduce the linklet from any host racket.

How the script works

• Detects the host racket version and pins the racket source to the matching tag (override with an explicit ref argument).
• Shallow-clones racket/racket at that tag, builds zuo, and runs the expander's own expander-src demod target using the host racket as the flattening engine — no full Chez/Racket build (~50s).
• Copies the result over expander/expander.rktl after a structural (single-linklet) sanity check; fails loudly on missing prerequisites (git / C compiler / racket / compiler/cm).

scripts/gen-expander.sh            # match installed racket
scripts/gen-expander.sh v9.3       # force a specific ref

Notes

• The flattened linklet's exported interface is unchanged (71 exports, none added/removed); only internal definitions differ (body 1930 → 2003 forms).
• Verified: debug build is green (13/13 tests pass), and norac runs normal linklets fine.
• norac still cannot parse the full expander linklet — but that is pre-existing (the old 2023 file fails identically), not a regression from this change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.19%. Comparing base (ee7d9a4) to head (3662734).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #74   +/-   ##
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  Coverage   73.19%   73.19%           
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  Files          24       24           
  Lines        2108     2108           
  Branches      290      290           
=======================================
  Hits         1543     1543           
  Misses        565      565           

Flag	Coverage Δ
integration	73.19% <ø> (ø)

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