plan: tick Phase 5 Tests+Commit tasks — string-buffer complete

17 tests written inline with the implementation step. All 17 pass
on OCaml and JS. Phase 5 fully done as d98b5fa2.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plans/agent-briefings/primitives-loop.md

@@ -156,8 +156,13 @@ Fix O(n²) string concatenation in loops across Lua, Ruby, Common Lisp, Tcl.
       `string-buffer-length` to primitives. OCaml: `Buffer.t` wrapper. JS: array+join.
       Also: string-buffer? predicate; SxStringBuffer._string_buffer marker for typeOf/dict?
       exclusion; inspect case in sx_types.ml. 17/17 tests OCaml+JS.
-- [ ] Tests: 15+ tests.
-- [ ] Commit: `spec: string-buffer primitive`
+- [x] Tests: 15+ tests.
+      17 tests written inline with Spec+OCaml step: construction, type-of, empty/length,
+      single/multi-append, append-returns-nil, empty-string-append, reuse-after-to-string,
+      independence, loop-building, CSV-row, unicode, repeated-to-string, join-pattern.
+      17/17 OCaml+JS.
+- [x] Commit: `spec: string-buffer primitive`
+      Committed as d98b5fa2 — all work in one commit (OCaml type + primitives + JS + spec + 17 tests).
 
 ---
 
@@ -678,6 +683,7 @@ Brief each language's loop agent (or do inline) after rebasing their branch onto
 
 _Newest first._
 
+- 2026-04-26: Phase 5 complete — string buffer fully landed (d98b5fa2). 17 tests, 17/17 OCaml+JS. Phase 6 (ADTs) next.
 - 2026-04-26: Phase 5 Spec+OCaml+JS step done — StringBuffer of Buffer.t in sx_types.ml; make-string-buffer/append!/->string/length/string-buffer? in sx_primitives.ml; SxStringBuffer with _string_buffer marker + typeOf/dict? fixes in platform.py; JS rebuilt. 17/17 tests OCaml+JS.
 - 2026-04-26: Phase 4 complete — coroutine primitive fully landed (4 commits: spec library + OCaml verified + JS pre-load + 27 tests). Phase 5 (string buffer) next.
 - 2026-04-26: Phase 4 Tests step done — 27 tests total (10 new: state field inspection, yield-from-helper, initial-arg-ignored, mutable-closure, complex-values, round-robin, factory-no-state, non-coroutine-error). 27/27 OCaml+JS.