Implement Comprehensive Benchmarking for Runtime Extrinsics

## Overview
Implement proper benchmarking for all runtime extrinsics to ensure accurate transaction fee calculation.

## Background
Benchmarking files exist (`node/src/benchmarking.rs`, `runtime/src/benchmarks.rs`) but contain minimal implementation. Without proper benchmarks, transaction fees may be incorrectly calculated, leading to potential DoS vectors or unfair pricing.

## Tasks
- [ ] Implement benchmarks for all runtime extrinsics
- [ ] Add benchmarks for contract calls
- [ ] Run benchmarks on target hardware
- [ ] Update weight calculations based on benchmark results
- [ ] Document benchmarking procedures
- [ ] Set up automated benchmark regression tests

## Related
- Code review finding: M5

## Priority
MAJOR - Required for accurate fee calculation

## Estimated Effort
3-5 days

Start by reading node/src/benchmarking.rs and runtime/src/benchmarks.rs to inventory the existing minimal coverage and determine which runtime extrinsics and contract calls are missing. Run the available benchmarks on target hardware, then use the results to update weights; completion also requires documented procedures and automated regression coverage.

Written by the indexing model from the issue text.