fix(tdd): post-merge Copilot round-4 fixes — class-invariant rewrite + fixture clarity

[intel352]

Summary

Cherry-pick of commit 911c62e, which was not captured in the PR #5 squash merge (f1baa9a). Contains 4 files changed across the test-driven-development skill:

• SKILL.md — add flow-note linking Verify Regression Invariant prose to diagram node (verify_green → verify_invariant → refactor); fix class-invariant revert-and-restore guidance (Important finding: PR skill: test-driven-development — Verify Regression Invariant + class invariant coverage #5 shipped wrong instruction)
• scenario.md — clarify Sum spec: 0 on empty input is correct; the actual bug is 0 on single-element input (off-by-one in loop)
• old-flow.md — reframe "returns 0 for empty" as correct behavior, not a symptom; describe the panic as a regression from correct to crashing
• new-flow.md — fix indentation: Edge-case sweep is under Class invariant heuristic check (was at wrong level)

Key fix — class-invariant guidance (Important)

PR #5 shipped this in SKILL.md:

Both must fail when the class invariant is broken and pass when it is restored.

That is wrong — "both must fail" implies ALL sibling rows fail when you revert ONE method. The correct behavior is:

Old (wrong):

Both must fail when the class invariant is broken

New (correct):

revert only the previously-broken method so the overall test fails (its row fails;
correct siblings still pass), then restore it so the full table passes again.
Optionally, temporarily introduce the same invariant violation into a known-good
sibling to prove its row would also fail, then restore both and confirm the table
is green.

Runtime launch transcript

Step 1b trigger check: documentation-only PR (skill prose + fixture markdown). No build config, deployment config, version pins, startup config, migrations, or plugin loading paths touched. Step 1b: NOT triggered.

Step 1c trigger check: no version pins in diff. Step 1c: NOT triggered.

Verdict: PASS (doc-only; neither runtime-launch nor version-skew steps apply)

Verify Regression Invariant

Not applicable — documentation-only, no code paths. Prose correctness verified by reading old vs. new:

• Old: "Both must fail" — logically incorrect for table-driven test reverting one method
• New: "its row fails; correct siblings still pass" — matches actual test behavior

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Updates the test-driven-development skill documentation and fixtures to correct previously incorrect class-invariant regression guidance and to clarify the narrow-regression fixture narrative (what’s “correct behavior” vs the actual bug/regression).

Changes:

• Link “Verify Regression Invariant” prose to the corresponding diagram node/path (verify_green → verify_invariant → refactor).
• Correct the class-invariant “revert-and-restore” instruction to reflect expected table-driven test behavior (only the reverted method’s row fails; correct siblings pass).
• Clarify the narrow-regression fixture story: empty input returning 0 is correct; the regression is an empty-slice panic, and the original bug is single-element returning 0.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
skills/test-driven-development/SKILL.md	Fixes/clarifies Verify Regression Invariant + class-invariant revert/restore guidance and ties prose to the diagram node.
skills/test-driven-development/test-fixtures/narrow-regression/scenario.md	Clarifies the intended spec vs the actual off-by-one bug in the fixture scenario.
skills/test-driven-development/test-fixtures/narrow-regression/old-flow.md	Reframes the “hidden problem” as a regression from correct empty handling to a crash.
skills/test-driven-development/test-fixtures/narrow-regression/new-flow.md	Fixes list indentation so Edge-case sweep sits under the intended heuristic section.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 4 changed files in this pull request and generated no new comments.

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