Increase test coverage to ~99.5%

[david-littlefarmer]

Summary

• The formatter package had almost no tests (7.5% coverage, only comments_test.go from Fix comment parsing to respect word boundaries around '#' #18). Added formatter/errors_test.go, formatter/formatter_test.go, formatter/processor_test.go, and extended comments_test.go, covering the section-parsing state machine (formatLine/parseSection/processLines), comment/error printing, method-argument formatting, and the comment/hash word-boundary logic from Inconsistent pathological comment parsing #17.
• Fixed the dead/unused testHelpFlag helper in main_test.go (lowercase name meant it never ran, and its logic was broken anyway) and replaced it with real tests, including subprocess-based tests for the os.Exit/log.Fatalf code paths that would otherwise kill the test binary (and its coverage profile) if exercised in-process.
• Added make coverage (prints per-function %) and make coverage-html targets.
• Added coverage.html to .gitignore.

Coverage

Package	Before	After
formatter	7.5%	99.7%
main (root)	54.7%	98.4%
total	15.2%	99.5%

Two spots remain below 100%, both intentionally so:

• formatter/processor.go's annotation branch len(parts) < 2 is unreachable dead code — parseSection only routes to that branch when the line already starts with @, so the split can never return fewer than 2 parts.
• main()'s log.Fatalf error branch calls os.Exit, which can't be safely triggered in-process (it would abort the test binary before it writes its own coverage profile). It's verified behaviorally via subprocess tests instead (TestSubprocess_*), which don't count toward this package's in-process coverage percentage.

Test plan

• go build ./...
• go test ./... (all pass)
• make coverage — 99.5% total
• gofmt -l . / go vet ./... clean