Maximize overall project test coverage

[jackfirth]

Now that I've set up Coveralls, it's easier to tell where the test coverage gaps are. Resyntax currently has about 94% coverage but there's several specific files that have much worse than that. Let's see how close to 100% we can get.

[jackfirth]

Current state as of master (4e4ffb5): 95.36% on Coveralls, 824 missed lines out of 17,755. The misses are concentrated in tool plumbing rather than refactoring rules — twelve files account for ~640 of the 824 lines, and most rule modules are already at 95–100%. Filed one issue per gap, each sized for a single small PR:

Missed	Coverage	File	Issue
114	68.6%	cli.rkt (run/output paths)	#818
99	76.8%	main.rkt (mostly covered via #818/#819)	—
72	84.6%	grimoire/syntax-path.rkt (error branches)	#816
69	59.9%	private/refactoring-result.rkt (pure functions)	#815
69	76.8%	test/private/rackunit.rkt (failure reporting)	#821
51+45	60.5%	UTS checker + renderer	#822
48	61.0%	private/github.rkt (env vars block testing)	#820
33	82.8%	test.rkt (syntax error paths, part of #821)	#821
29	23.7%	private/run-command.rkt	#817
16	72.9%	private/git.rkt	#819

#815, #816, and #817 are mechanical (~170 lines of pure code and error branches). #818 is the biggest single win. #815–#819 together would put the project at roughly 97%. A realistic ceiling is around 98% — the long tail is defensive raises and debug-log branches with poor value per test.