docs: log the WASM/emscripten-guard lesson in MISTAKES.md

Author: pmarreck

MISTAKES.md

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+## 2026-06-27 — src/printable_binary.c is compiled by THREE toolchains; clang-clean ≠ all-clean
+Added container support to the standalone C (`src/printable_binary.c`) and the
+`test-container-c` check (clang `-O3 -Wall -Wextra`) passed — but I pushed without
+building the WASM/APE targets. The emscripten build broke: my container block
+called `buffer_free`, which is `#ifndef __EMSCRIPTEN__`-guarded (so it doesn't
+exist in WASM), failing `printableBinaryWasm` and the `default`/`nix`/`All Garnix`
+aggregates that depend on it.
+**Lesson:** `src/printable_binary.c` is built by clang (native), emscripten (WASM),
+AND cosmocc (APE) — each with different defines/guards. Before pushing changes to
+it, build `nix build .#printableBinaryWasm` (and `.#printableBinaryApe`) locally,
+not just the clang container check. A native-clang pass does NOT catch
+`__EMSCRIPTEN__`-guarded-symbol regressions.