X3DH session establishment over existing prekeys

[codebestia]

Description:
Wire X3DH to the existing bundle endpoint (#110): identity key + signed prekey + one-time prekey already match X3DH inputs. Verify the server's atomic OTP consumption (#110) satisfies Signal's one-time-use requirement. Adjust registrationId usage (#103) as the library expects.

Acceptance criteria:

• X3DH handshake succeeds using stored prekeys
• OTP consumed exactly once per new session
• Falls back to signed-prekey-only when OTPs exhausted

[otobongdev]

@otobongdev has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I can fix this issue

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[Mkalbani]

@Mkalbani has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainers. I'm a Web3 Fullstack dev. I've worked on a similar work. Would be done before the ETA. Thank you.

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[fairbid01]

@fairbid01 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would like to work on this issue

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