Fix embedded mnemonic share index bits

[Randy-sin]

Fixes #55

This keeps embedded share indexes within the checksum bit width that can actually be stored by the mnemonic.

Changes:

• Validate splitMnemonic(..., { embeddedIndex: true }) so total shares cannot exceed the mnemonic checksum capacity.
• Use Math.ceil(Math.log2(total + 1)) instead of Math.ceil(Math.log2(total)) + 1 for the share-index mask width.
• Add regression coverage for a 12-word mnemonic split into 15 shares, ensuring embedded indexes stay in 1..15 and combining still restores the original mnemonic.
• Add a rejection test for trying to embed 16 shares into a 12-word mnemonic.

Validation:

• npm test (85 passing)
• npm run build

Note: I kept the patch intentionally small because this touches wallet recovery behavior.