Add zero-confirmation risk analysis

[SadmanPinon]

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

Summary

• Adds a Zero-Confirmation Risk Analysis subsection to the payment verification guidance.
• Explains why mempool visibility is local policy evidence rather than consensus.
• Distinguishes unconfirmed double-spend attempts from chain-committed double spends.
• Covers stuck transactions, RBF/CPFP fee incentives, risk monitoring, and confirmation/manual-review policies.
• Clarifies that CLTV/CSV can constrain contract paths but cannot make ordinary zero-confirmation payments final.

Verification

• git diff --check
• PATH=/tmp/developer-bitcoin-venv/bin:$PATH make clean html

The Sphinx build succeeds with the repository's two existing warnings only: conf.py language fallback and glossary.rst:647 glossary spacing. This PR does not add new warnings.

Demo / Review Proof

• Short demo video: https://github.com/SadmanPinon/bounty-proof-assets/blob/main/demos/bitcoin-1216-demo.mp4

Bounty

This targets the 25,000 bits / ~$250 USD bounty in bitcoin-dot-org/Bitcoin.org#1216. No payout address is included in this public PR; payout details can be supplied if this entry is selected.