feat(tdd): add TPP + ZOMBIES next-test selection to the /tdd skill

[Hyperman012]

Summary

The /tdd skill's loop says to write a failing test and make it pass; it does not say which test to write next. This PR adds the two complementary test-selection heuristics as a new progressive-disclosure reference for the skill:

• Transformation Priority Premise (TPP), Robert C. Martin's ranked list of code transformations, simplest first. Pick the test whose passing requires the highest-priority transformation still available.
• ZOMBIES, James Grenning's Zero, One, Many, Boundaries, Interface, Exceptions, Simple mnemonic, as the concrete ordering for the same simplest-first selection.

What's included

• han-coding/skills/tdd/references/test-selection.md (new): the canonical 14-transformation table, the "as the tests get more specific, the code gets more generic" principle, the decision-point rule with the bubble-sort/quicksort demonstration, the ZOMBIES table and Z-O-M ordering rule, diagnostic signals, a worked sum() example, and sources (Martin, Grenning, Langr).
• han-coding/skills/tdd/SKILL.md: three hooks at the skill's existing seams. Step 2 orders a behavior's candidate tests Zero -> One -> Many; Step 3 uses the ranking to pick among qualifying items (and to detect a missing smaller test); the frontmatter description gains TPP and ZOMBIES trigger vocabulary.
• docs/skills/han-coding/tdd.md: a Key concepts bullet and two Sources entries, per the long-form-doc coverage rule.

Alignment with the skill's existing discipline

The reference reconciles the heuristics with the skill rather than importing them wholesale:

• User value picks the behavior; TPP picks the test. Step 2's outside-in user-value ordering and TPP's simplest-first ordering answer different questions and do not compete.
• TPP is how you find Beck's "one step test" that Step 3 already asks for.
• The gears stay small because the tests were chosen small. Fake It is nil -> constant; Triangulate adds the example that forces constant -> scalar.
• A confirming first-run pass maps onto the observed-failure gate's existing "behavior already exists" diagnosis instead of fighting it.

Field test

Exercised by driving the modified skill through the Prime Factors kata (Ruby/minitest), with per-cycle logging and one commit per cycle for an auditable order. Results:

• Test order emerged as the canonical TPP sequence: of(1) (Z, nil -> constant), of(2) (O, unconditional -> if), of(3) (O, constant -> scalar via triangulation), of(4) (M, if -> while), of(9) (M, divisor generalization), of(12) (confirming test).
• The confirming test on 12 passed on first run; the observed-failure gate tripped and the "behavior already exists" diagnosis applied, exactly as the reference predicts.
• The classic nested-while algorithm emerged without being planned. Final suite: 6 runs, 6 assertions, 0 failures, 0 errors, 0 skips.
• Two ambiguities the field test surfaced in the new material are fixed in this PR: a tie-break rule for candidates demanding the same transformation, and a note that one test may force a small cluster of transformations (compare by the deepest-ranked member).

Two pre-existing observations from the same run, out of scope here but noted for maintainers: on a first-run pass the "cross it off and pick the next one" instruction skips that cycle's otherwise non-skippable refactor statement, and Step 5 gives no instruction for projects with no lint or build command configured.

Credit and sources

Ported from Dale Stewart's nw-tpp-methodology skill for nWave (nWave-ai/nWave#67) and his write-up "The Next Test" (https://meresoftware.substack.com/p/the-next-test), adapted to this skill's loop, reference conventions, and writing voice. Primary sources: Martin's "The Transformation Priority Premise", "Transformation Priority and Sorting", and "The Cycles of TDD"; Grenning's "TDD Guided by ZOMBIES"; Langr's Modern C++ Programming with Test-Driven Development.

🤖 Generated with Claude Code

[mxriverlynn]

Hi @Hyperman012 - this is great! i hadn't heard about these forms of test selection, but after reading everything in this PR, it makes sense. I think somewhere in my career, i ended up picking up some informal version of these ideas.

i'll give this update a few runs over the next few days and let you know if i have any feedback. thank you!

[Hyperman012]

Hi @Hyperman012 - this is great! i hadn't heard about these forms of test selection, but after reading everything in this PR, it makes sense. I think somewhere in my career, i ended up picking up some informal version of these ideas.

i'll give this update a few runs over the next few days and let you know if i have any feedback. thank you!

Absolutely! I love this stuff! and i've been loving these Han skills. so thank you for publishing so i can add on!