KIP-1367 — Reserve apiKey values for private use

Discussion Protocol

Reserves apiKey values [31744, 32767] for private use and adds a `--allow-custom` flag to `MessageGenerator` (opt-in via the `processMessages` Gradle task) so specs in that range fail the build unless explicitly enabled. Contributors prototyping new message types risk picking an apiKey that another proposal later claims, and today the only workaround is informally squatting on a high unused value.

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