Fix embedded mnemonic share index bit sizing

[Michealkene]

Fixes #55.

Summary:

• Derive embedded share index bit capacity from the BIP39 entropy/checksum size.
• Use that capacity by default in __split_secret, so 12-word mnemonics only generate encodable share indexes.
• Keep validation when the requested total share count exceeds the embedded index capacity.
• Add regression coverage for a 12-word embedded-index split/combine path and the 16-share overflow case.

Tests:

• git diff --check
• Local smoke coverage for the affected split/combine path passed in the earlier work session.
• Full npm test was not completed locally because dependency install timed out and mocha was unavailable in this checkout.