M1: core path escaping, order-preserving index value encoding, errors

Scaffold TypeScript/Vitest project, add DESIGN.md (brief) and CLAUDE.md.
Property tests cover escape round-trips, separator-freedom and order preservation.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Qxbkp4tA7G5SCWjzWuxmE

CLAUDE.md

@@ -0,0 +1,20 @@
+# PotentialStorage
+
+Layered TypeScript storage library: KV → Events → Documents. The full design, milestones and
+implementation decisions are in [DESIGN.md](./DESIGN.md) — read it before changing anything.
+Record new decisions in DESIGN.md section 12.
+
+## Commands
+
+- `npm test` — run all tests (Vitest; includes `*.test-d.ts` type tests)
+- `npm run typecheck` — `tsc --noEmit` over src, test and bench
+- `npm run bench` — `vitest bench`
+- `npm run build` — emit `dist/`
+
+## Rules
+
+- No runtime dependencies in `src/`. Dev dependencies only.
+- Drivers stay flat and string-keyed; all path/index/escaping logic lives in the node classes.
+- Self vs child dispatch is by `arguments.length`; a passed-but-`undefined`/`null` key throws.
+- Multi-result reads are `AsyncIterable`; don't build large arrays internally.
+- Keep tests green and commit per milestone. Ask the owner before pushing.