Fall Out Bug Sdp Tdd

@tdd (INTERNAL)

TDD discipline. Called by @build, not users.

Cycle

1. RED — Write failing test first. Run test suite (see Quality Gates in AGENTS.md) — must FAIL
2. GREEN — Minimal implementation. Run test suite (see Quality Gates in AGENTS.md) — must PASS
3. REFACTOR — Improve code. Run test suite (see Quality Gates in AGENTS.md) — still PASS
4. COMMIT — Save state

Exit When

• All AC met
• Test suite passes (see Quality Gates in AGENTS.md)
• Static analysis passes (see Quality Gates in AGENTS.md)

Example (Go)

// RED: test first
func TestEmailValid(t *testing.T) {
    v := NewValidator()
    if !v.IsValid("a@b.com") { t.Error("expected valid") }
    if v.IsValid("x") { t.Error("expected invalid") }
}
// Run: FAIL (undefined NewValidator)

// GREEN: minimal impl
func NewValidator() *V { return &V{} }
func (v *V) IsValid(s string) bool { return strings.Contains(s, "@") }
// Run: PASS

// REFACTOR: improve, tests still pass

For Go refactors, prefer modern stdlib idioms from @go-modern when they preserve behavior.

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