Refactoring and testing

[jippi]

• simplified tui package
• refactored most internals to be more resilient and testable
• greatly expanded test coverage
• aligned naming of various types across the codebase

[codecov]

Codecov Report

Attention: 161 lines in your changes are missing coverage. Please review.

Comparison is base (28a0848) 48.17% compared to head (25a2079) 58.93%.

Files	Patch %	Lines
cmd/validate/validate.go	3.84%	25 Missing ⚠️
cmd/update/update.go	4.16%	23 Missing ⚠️
pkg/validation/explain.go	28.12%	23 Missing ⚠️
pkg/tui/printer.go	67.30%	17 Missing ⚠️
pkg/render/output_colorized.go	0.00%	13 Missing ⚠️
cmd/value/value.go	7.69%	12 Missing ⚠️
pkg/ast/document.go	45.00%	9 Missing and 2 partials ⚠️
pkg/ast/upsert/upsert.go	77.77%	4 Missing and 2 partials ⚠️
pkg/tui/theme.go	81.81%	5 Missing and 1 partial ⚠️
cmd/fmt/fmt.go	16.66%	5 Missing ⚠️
... and 8 more

Additional details and impacted files

@@             Coverage Diff             @@
##             main       #7       +/-   ##
===========================================
+ Coverage   48.17%   58.93%   +10.76%     
===========================================
  Files          57       58        +1     
  Lines        2761     2849       +88     
===========================================
+ Hits         1330     1679      +349     
+ Misses       1370     1080      -290     
- Partials       61       90       +29     

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