Rework shielded transactions

[l0r1s]

Summary

Companion PR to the opentensor/polkadot-sdk#6

Update MEV shield on subtensor — a system that encrypts transactions client-side so their contents
are hidden from the mempool, preventing front-running and sandwich attacks. Block authors decrypt
shielded transactions at block building time using ML-KEM-768 + XChaCha20-Poly1305.

Changes

pallet-shield

• announce_next_key inherent: rotates CurrentKey ← NextKey each block and publishes the
  upcoming block author's ML-KEM public key so users can encrypt for them
• submit_encrypted extrinsic: accepts an encrypted ciphertext wrapper from users
• try_decode_shielded_tx / try_unshield_tx: runtime helpers called by the block
  proposer (via the ShieldApi runtime API) to detect, decrypt, and decode shielded transactions
• CheckShieldedTxValidity transaction extension: validates key_hash against
  CurrentKey/NextKey during block import; pool validation only checks ciphertext structure to
  avoid spurious rejections from stale views
• FindAuthors trait for resolving current and next block author

stp-io (new crate — host functions)

• mlkem768_decapsulate: ML-KEM-768 decapsulation via the ShieldKeystore extension
• aead_decrypt: XChaCha20-Poly1305 decryption via the ShieldKeystore extension
• Exported as SubtensorHostFunctions for registration in the node

Node integration

• ShieldKeystore is created in service.rs and threaded through to the proposer and inherent
  providers
• ShieldInherentDataProvider added to Aura and Babe consensus configurations
• KeyRotationService generates a new ML-KEM keypair on each own-block import
• SubtensorHostFunctions registered in the client executor

Runtime integration

• ShieldApi runtime API implemented (try_decode_shielded_tx, try_unshield_tx)
• CheckShieldedTxValidity added to the transaction extension pipeline

Tests

• pallet-shield: key rotation, try_decode_shielded_tx, try_unshield_tx, depth-limit
  protection, inherent creation
• CheckShieldedTxValidity extension: key_hash matching against CurrentKey/NextKey, malformed
  ciphertext rejection, pool vs in-block source behavior

[github-actions]

🚨🚨🚨 HOTFIX DETECTED 🚨🚨🚨

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