tests: add coverage to headless units

[bjacobgordon]

Phases

1. pre-requisites
   • linter(vitest): preps for first unit tests #510
   • refactor(library): flattens contents of "/customTypes" into parent #515
   • tests(vitest): adds support for coverage analysis #528
   • ci(node): enables unit testing #531
   • build(IDE): nests test files under their unit #537
2. Perform "vertical attack" to gauge checkpoints:
   1. Take easiest functional unit from "nothing" to "covered": library/math -> greatestCommonDivisor
      • refactor(library/math): preps directory for tests #518
      • tests(library/math): applies template for functional suite to greatestCommonDivisor #533
      • tests(library/math): stubs complete coverage for greatestCommonDivisor #532
      • fix(library/math): guards against inoperable operands in greatestCommonDivisor #534
      • refactor(library/math): separates concerns of "number taxonomy" from isOperable test suite #539
      • fix(library/math): guards against infinite operands in greatestCommonDivisor #538
      • tests(library/math): clarifies labels for greatestCommonDivisor suite #543
      • tests(library/math): clarifies test parameterization in greatestCommonDivisor suite #542
      • fix(library/math): guards against fractional operands in greatestCommonDivisor #541
      • tests(library/math): completes spec for in-scope failure mode of greatestCommonDivisor #545
      • tests(library/math): completes "sad path" coverage for greatestCommonDivisor #544
      • fix(library/math): adds support for negative operands to greatestCommonDivisor #567
      • tests(library/math): specs operand order for greatestCommonDivisor #568
      • refactor(library/math): calls out assumed operand order in greatestCommonDivisor #569
      • tests(library/math): specs handling of 0 in greatestCommonDivisor #570
      • tests(library/math): specs preservation of operand identity in greatestCommonDivisor #571
      • tests(library/math): specs how co-primes should be handled in greatestCommonDivisor #572
      • tests(library/math): completes coverage of greatestCommonDivisor #573
      • refactor(library/math): simplifies implementation in greatestCommonDivisor #574
      • refactor(library/math): documents alternative terms for "greatest common divisor" #575
   2. Take easiest class from "nothing" to "covered": NonTrivialString
      • prep directory to allow separation of test suites, performing useful refactors prior to creating any test suites
      • stub empty class-based suite
      • seed test template for class-based suite
      • stub complete coverage outline with it.todo
      • cover parsing rejection of empty strings
      • cover parsing rejection of null inputs in nullable parser
      • clarify error messages for parsing failures
      • cover successful parsing of valid strings
      • cover nullableParsedFrom success scenarios
      • cover equality comparison behavior
      • cover instance concatenation
      • cover static concatenation
      • cover inheritance
      • cover interface implementation
      • cover concatenation properties and laws
      • cover parsing consistency and idempotence
      • complete comprehensive coverage validation
      • optimize test organization and clarity
3. Perform "horizontal attack" using checkpoints
   • tests: stub suite for all units #1051
4. Enforce
   • warn when coverage thresholds are not met