Document unconfirmed payment risk analysis

[alan747271363-art]

Addresses bitcoin-dot-org/Bitcoin.org#1216.

Summary:

• Adds a focused "Unconfirmed Transaction Risk Analysis" subsection to the payment verification guide.
• Explains that mempool observation is local relay-policy evidence, not network consensus.
• Distinguishes unconfirmed conflicts from chain-committed double spends.
• Covers stuck low-fee transactions, RBF/CPFP fee-bumping limits, and why CLTV/CSV do not create instant settlement.

Verification:

• git diff --check
• python -m sphinx -E -b html . _build/html

Bounty note:

• This PR intentionally does not include public payout information. I can provide bounty/payment details privately if this submission is selected.