Use raw signing by default

[mateeullahmalik]

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[roomote]

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This commit simplifies the signing approach by switching from ADR-36 to raw signing throughout the codebase. Key changes:

• SignIndexB64 now signs the base64-encoded payload (indexB64) instead of the raw JSON string, aligning with how SignLayoutB64 works
• Removed ADR-36 related functions (adr36SignerForKeyring, CreateSignaturesWithKeyringADR36WithSigner, CreateSignaturesWithKeyringADR36, SignADR36String)
• BuildCascadeMetadataFromFile and GenerateStartCascadeSignatureFromFileWithSigner now use raw signing

Backward compatibility is maintained since VerifyIndex and VerifyStringRawOrADR36 already handle both raw and ADR-36 signature formats.

Previous reviews

• 8f61b0b: Review #1
• e9b46a3: Review #2

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