Fix embedded mnemonic share index bit sizing

[unworthyzeus]

Summary

• Size embedded mnemonic share indexes with ceil(log2(total + 1)) so 12-word mnemonics can use all valid 4-bit indexes, including 9-15 shares.
• Reject embedded-index splits when the requested share count cannot fit in the mnemonic checksum bits, with the same clear max-share limit used by Wallet.
• Add regression coverage for 12-word embedded-index shares: 15 shares recover successfully, while 16 shares fail before generating unusable shares.

Notes

This is limited to embeddedIndex mode. Non-embedded splitMnemonic still keeps share indexes in the returned object keys.

Tests

pm test

Addresses #55