Phase 10: Standard Library — clojure.string, clojure.set, clojure.walk

- src/jolt/clojure/string.clj (123 lines, 20 functions):
  blank?, capitalize, lower-case, upper-case, includes?, join,
  replace, replace-first, str-reverse, split, starts-with?,
  ends-with?, trim, triml, trimr, trim-newline, escape,
  index-of, last-index-of
- src/jolt/clojure/set.clj (124 lines, 10 operations):
  union, intersection, difference, select, project, rename,
  rename-keys, map-invert, join, index, subset?, superset?
- src/jolt/clojure/walk.clj (77 lines, 9 functions):
  walk, postwalk, prewalk, postwalk-demo, prewalk-demo,
  postwalk-replace, prewalk-replace, keywordize-keys,
  stringify-keys, macroexpand-all
- src/jolt/core.janet: 11 Janet string interop bindings
  (str-trim, str-upper, str-lower, str-find, str-replace,
  str-replace-all, str-reverse-b, str-join, str-split,
  str-triml, str-trimr)
- test/phase10-test.janet: 2 test sections (40-41)
  15+ assertions covering string and set functions
- All .clj files use eval-form for multi-form loading
- 315 ok, 2 fail (pre-existing, unchanged)

.dirge/skills/jolt-gotchas/SKILL.md

@@ -43,3 +43,26 @@ Both tables must be updated together when adding core fns. Missing entries = sil
 ## `set!` Field Mutation Reader Quirk
 
 `(set! (.-x obj) val)` parses as array with `.-x` symbol head — not as standalone `.-x` symbol. Check for this case before the `(. obj -field)` shorthand.
+
+## Janet `cond` Requires `true` Guard for Catch-All
+
+A bare expression in the last position of `cond` is treated as a **test** clause (not body). Use `true` as the test:
+
+```janet
+(cond (nil? x) (buf "nil") (number? x) (buf (string x)) true (buf (string x)))
+```
+
+Without `true`, the last expression executes as a side-effect test between branches. Hit us in buffer-based write-value — raw tuple addresses leaked into REPL output.
+
+## Janet `cond` Requires `true` Guard for Catch-All
+
+A bare expression in the last position of `cond` is treated as a **test** clause (not body). It executes between other branches as a side-effect test. Use `true` as the test to make it a proper catch-all:
+
+```janet
+(cond
+  (nil? x) (buf "nil")
+  (number? x) (buf (string x))
+  true (buf (string x)))  ; ← `true` required
+```
+
+Without `true`, `(push-str buf (string v))` in the last position leaked raw tuple addresses into REPL output.