fix: generate correct BIP39 entropy size in Mnemonic::new()

[shivv23]

Description

Mnemonic::new() was always generating 32 bytes of entropy regardless of the requested word count, with an explicit TODO expressing uncertainty about the approach.

Changes

• Generate entropy of the correct size based on word count (16 bytes for 12-words, 24 for 18-words, 32 for 24-words)
• Replace the redundant generate_with_entropy → o_string() → parse_in() round-trip with a direct rom_entropy() call
• Remove unused imports (GeneratableKey, GeneratedKey, BareCtx)

Testing

• Added est_mnemonic_generation_word_count to verify generated mnemonics have the correct word count