fix: reject single-share Shamir splits

[slashdevcorpse]

Summary

• Reject Shamir splits with thresholds below 2, matching the restore path that requires at least two points.
• Update the mnemonic and helper tests so threshold-1 requests fail at split time instead of producing share sets that cannot be recovered through the existing API.

Validation

• Focused affected Mocha tests for initialization, mnemonic split/combine, embedded indexes, GF256, and secret splitting
• npm test
• git diff --check

Bounty payout address if accepted: bc1qssrlr7qu0kq2kmf7arwznpuad0aw5wr9ls7c3q

[Copilot]

Pull request overview

This PR prevents creation of unrecoverable Shamir share sets by rejecting threshold-1 splits at split time, aligning behavior with the existing restore path (which requires at least two points).

Changes:

• Add an explicit threshold < 2 guard in __split_secret.
• Update mnemonic splitting tests to expect failure for threshold=1 instead of producing shares.
• Add a test asserting raw secret splitting rejects threshold=1.

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated no comments.

File	Description
test/jsbtc.test.js	Updates/extends tests to assert threshold-1 splits throw "Minimum 2 shares required".
src/functions/shamir_secret_sharing.js	Adds a runtime guard rejecting threshold < 2 in __split_secret.

Files not reviewed (1)

• test/jsbtc.test.js: Language not supported

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