feat(add): idempotency key + single network retry

[agalwood]

What

• motrix add now generates a fresh idempotencyKey (UUID) per invocation on all three variants (url/magnet/torrent)
• On a network-level failure (EXIT.NETWORK — fetch threw, the response may have been lost after delivery) the same request is retried exactly once with the same key; JSON-RPC errors never retry; no other command gains retry
• rpcCall adds an upgrade hint when a host rejects with -32602 InvalidParams (protocol drift: older Motrix rejecting the new field)
• @motrix/mdxp bumped ^0.3.0 → ^0.5.0

Contract: MDXP idempotency & retry contract (§8.5) — a keyed replay returns the first submission's task snapshot, so the single retry can never mint a duplicate task. Requires Motrix with @motrix/mdxp@0.5.0 schema for motrix add; older hosts reject loudly with the new upgrade hint.

Verification

• pnpm lint / pnpm typecheck / pnpm build clean; 221/221 tests
• New tests: retry reuses the identical key across both attempts; exactly one retry then EXIT.NETWORK; JSON-RPC errors surface without retry; -32602 hint preserves the original error as data

Note before next npm publish: bump package version (currently 0.4.0).