tools: add Bitflash mining calculator

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+# Bitflash mining calculator
+
+`scripts/bitflash-mining-calculator.py` is a local WhatToMine-style calculator
+for Bitflash.
+
+It does not fetch market prices, does not talk to exchanges, and does not
+change consensus. Price is a manual input until Bitflash has a reliable public
+market.
+
+## Examples
+
+Estimate from network hashrate:
+
+```bash
+python3 scripts/bitflash-mining-calculator.py \
+  --hashrate 21 --hashrate-unit kh/s \
+  --network-hashrate 1.56 --network-hashrate-unit mh/s \
+  --watts 140 --kwh-cost 0.10 \
+  --price 0.001 --pool-fee 1
+```
+
+Estimate from Bitflash difficulty:
+
+```bash
+python3 scripts/bitflash-mining-calculator.py \
+  --hashrate 21 --hashrate-unit kh/s \
+  --difficulty 45600 \
+  --watts 140 --kwh-cost 0.10
+```
+
+Machine-readable output:
+
+```bash
+python3 scripts/bitflash-mining-calculator.py \
+  --hashrate 21 --hashrate-unit kh/s \
+  --network-hashrate 1.56 --network-hashrate-unit mh/s \
+  --json
+```
+
+## Bitflash Difficulty Scale
+
+Bitflash does not use Bitcoin's SHA256 difficulty-1 constant for pool shares.
+The chain's easiest RandomX target is `bnProofOfWorkLimit = ~uint256(0) >> 12`,
+so difficulty 1 expects about `4096` hashes.
+
+The calculator uses:
+
+```text
+network_hashrate = difficulty * 4096 / block_time_seconds
+```
+
+At the default two-minute block target:
+
+```text
+network_hashrate = difficulty * 4096 / 120
+```
+
+If you already know network hashrate, pass `--network-hashrate` directly and
+the script derives the equivalent Bitflash difficulty for display.
+
+## Reward Estimate
+
+The expected daily reward is:
+
+```text
+blocks_per_day = 86400 / block_time_seconds
+miner_share = miner_hashrate / network_hashrate
+gross_btf_per_day = blocks_per_day * block_reward * miner_share
+net_btf_per_day = gross_btf_per_day * (1 - pool_fee_percent / 100)
+```
+
+Electricity is:
+
+```text
+electricity_per_day = watts / 1000 * 24 * kwh_cost
+```
+
+This is an expectation, not a promise. Solo mining variance can be very large,
+especially when the miner has a small share of the network.
+
+## Future Website Use
+
+The script is intentionally standalone so the same math can later back a
+website widget or status dashboard. Until a public price exists, a website
+calculator should keep price as a manual field and label fiat profit as
+speculative.