Revert automatic V3 routing (#105)

[kartojal]

Reverts #105.

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Note

Medium Risk
Changes default order signing/routing for V2 position IDs from forced V3 to server /version, which can cause mismatches or failed posts if the server version differs from what those assets require.

Overview
Reverts automatic Exchange V3 selection for protocol V2 position token IDs. Limit and market orders no longer infer exchange version from the token ID; when options.version is omitted, the client uses the cached /version response for every asset (same as legacy CTF routing).

Removes isV2PositionId and src/protocol.ts, drops resolveOrderVersion, and simplifies _retryOnVersionUpdate so migration retries are not skipped for V2 positions or explicit versions. README and CreateOrderOptions.version docs about auto-V3 are removed; related tests in orderVersion.test.ts and protocol.test.ts are deleted.

Callers that need Exchange V3 must pass version: 3 (or use createExchangeV3OrderFromAmounts where applicable).

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