mk: phase 5A — conda, soft-cut without onceo

conda-try mirrors condu-try but on the chosen clause it (mk-bind
(head-goal s) (rest-conj)) — all head answers flow through. condu by
contrast applies rest-conj to (first peek), keeping only one head
answer.

7 new tests covering: first-non-failing-wins, skip-failing-head, all-fail,
no-clauses, the conda-vs-condu divergence (`(1 2)` vs `(1)`), rest-goals
running on every head answer, and the soft-cut no-fallthrough property.

169/169 cumulative.

plans/minikanren-on-sx.md

@@ -124,8 +124,9 @@ Key semantic mappings:
       escapes to ground values for arithmetic or string ops
 - [ ] `matche` — pattern matching over logic terms (extension from core.logic)
       `(matche l ((head . tail) goal) (() goal))`
-- [ ] `conda` — soft-cut disjunction (like Prolog `->`)
-- [ ] `condu` — committed choice (already in phase 2; refine semantics here)
+- [x] `conda` — soft-cut: first non-failing head wins; ALL of head's answers
+      flow through rest-goals; later clauses not tried (`Phase 5 piece A`)
+- [x] `condu` — committed choice (Phase 2)
 - [ ] `nafc` — negation as finite failure with constraint
 - [ ] Tests: Zebra puzzle, N-queens, Sudoku via `project`, family relations via `matche`
 
@@ -155,6 +156,10 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-07** — **Phase 5 piece A — conda**: soft-cut. Mirrors `condu` minus
+  the `onceo` on the head: all head answers are conjuncted through the rest of
+  the chosen clause. 7 new tests including the conda-vs-condu divergence test.
+  169/169 cumulative.
 - **2026-05-07** — **Phase 4 piece B — reverseo + lengtho**: reverseo runs forward
   cleanly and `run 1`-cleanly backward; lengtho uses Peano-encoded lengths so it
   works as a true relation in both directions (tests use the encoding directly).