Security/shielded pool hardening

[nol4lej]

Security hardening of frame/shielded-pool, fixed change-by-change with tests. Requires a runtime upgrade (spec_version 2 → 3).

• Gate private_transfer on a registered, verified asset (the only path that skipped the asset state-machine).
• Reject asset-id collision instead of silently overwriting an existing asset.
• Bind the unsigned relayer field into the tx-pool provides tag.
• Error on unattributable relay fees instead of dropping them silently.
• Bound unsigned-tx longevity and claim_shielded_fees inputs.
• Price private_transfer per inserted leaf; make shield_batch use its benchmarked weight and reject empty batches.
• Regenerate all affected pallet weights.
• Harden the historic Merkle-root window (non-zero window enforced, atomic insertion, duplicate-aware eviction).
• Harden the pool-balance ledger invariant with a try_state check and defensive assertions.
• Reject non-32-byte recipient binding; tie the merkle capacity guard to the fixed tree depth.
• Unify Note.asset_id to u32 to match the circuit.