feat(payload): add per-tx and state change tracing spans

[decofe]

Adds two new debug_span! emissions to the payload builder for blockscope observability:

Per-tx execution span (execute_tx)

• Wraps each transaction execution in block_fill with tx_hash and gas_used attributes
• Enables blockscope to show per-tx breakdown within a block build

Block-level state changes span (state_changes)

• Emitted after finish_block with accounts_changed, storage_slots_changed, accounts_with_storage_changes
• Gated behind tracing::enabled! to avoid unnecessary iteration when debug tracing is off
• Unblocks RETH-594/595/596 (per-tx detail, LLM API, correlation graphs in blockscope)

Both follow the existing debug_span!(target: "payload_builder", ...) convention.

Prompted by: yk