reject with structured errors from the wasm bindings

[fabracht]

Closes #25.

Problem

The wasm bindings rejected with a plain string (JsValue::from_str(e.to_string())), so JS consumers had to substring-match Display text — e.g. String(err).startsWith("conflict for") — to tell a unique-conflict from anything else. Fragile, and it breaks silently if the message wording changes.

Change

Every failed Store method now rejects with a structured JS Error:

• stitch_err builds a js_sys::Error (message = the variant's Display) and attaches a stable kind plus entity/id for the variants that carry them, via Reflect::set.
• error_parts maps every stitch::Error variant to a kind (conflict, ownership, notFound, timeout, connectionClosed, sessionInvalid, mqtt, mqdb, config, serde, io, notInitialized, alreadyInitialized, scopeNotActive, unknownEntity) — exhaustive, so a future Error variant won't compile until it's mapped.
• The 34 self.inner.* call sites route through stitch_err; the serde/parse and non-serializable-input paths (JSON.stringify throwing on a BigInt/circular ref) carry kind: "invalidInput". No path rejects with a bare string.

Consumers can now write err.kind === "conflict" and delete their isUniqueViolation heuristic.

Breaking

Rejections change from strings to Error objects, so String(err) is now "Error: conflict for …". Callers matching the string must switch to err.kind/err.message. That makes it a minor bump — stitch-wasm 0.5.0 (left as [Unreleased] here; stitch-sync is untouched — the mapping lives entirely in the wasm crate).

Verification

• wasm32 check + clippy clean, incl. two #[wasm_bindgen_test]s asserting the mapping (conflict → kind/entity/id/message; timeout → kind only; generic → invalidInput). Browser tests execute in CI.
• Adversarial quorum (4 lenses + verify): surfaced one low finding — a non-serializable argument (BigInt/circular ref) rejected without a kind via JSON.stringify's raw throw — now fixed by routing it through invalidInput. Re-verified clean.