WIP: simplify interface to TWCC 1/4

[jech]

This is part 1/4 of a cross-repository series.

Please do not merge until the series is complete.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 71.33%. Comparing base (82bca40) to head (7ae31d0).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
registry.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #297      +/-   ##
==========================================
+ Coverage   71.26%   71.33%   +0.06%     
==========================================
  Files          80       80              
  Lines        4476     4486      +10     
==========================================
+ Hits         3190     3200      +10     
- Misses       1153     1154       +1     
+ Partials      133      132       -1     

Flag	Coverage Δ
go	71.19% <0.00%> (+0.06%)	⬆️
wasm	69.34% <0.00%> (+<0.01%)	⬆️

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