docs: restore the asciinema demo link and embed

[mallison031]

Description

The asciinema recording at https://asciinema.org/a/qqC0RysuCDBvfUXC is reachable again, so this restores every reference that was dropped when it was believed to have expired:

• README.md — the Demo link in the header nav and the asciicast embed at the top of Overview
• site/index.html — the full <section id="demo"> "See It in Action" block, restored verbatim from before its removal
• site/README.md and CHANGELOG.md — "asciinema demo embed" back in the landing-site description
• audit_and_roadmap.md — the Phase 12 "Demo Video" and Step 4 "Record a short demo video" entries

Two notes for the reviewer:

1. Duplicate roadmap bullets collapsed. Both demo entries in audit_and_roadmap.md appeared twice, with differently-worded removal notes ("temporarily removed while the asciinema recording is refreshed" vs. "removed as the link is no longer accessible"). That looks like a merge artifact. Each pair is collapsed back to a single restored bullet.
2. Line references updated. The roadmap cited README.md:9; the Dashboard link added since then shifted the demo down to README.md:10.

Related Issue

Reverts the documentation half of #224, which removed these references on the grounds that the link was dead.

Motivation and Context

#224 removed the demo link because it 404'd at the time. The recording resolves again now, and the landing page has been carrying a placeholder HTML comment where its demo section used to be. The .demo-box, .demo-player-wrapper, and .demo-fallback rules were never deleted from site/styles.css, so the restored markup is styled with no CSS changes needed.

How Has This Been Tested?

Docs and static-site only — no Rust code touched.

Link liveness verified before restoring rather than assumed:

• https://asciinema.org/a/qqC0RysuCDBvfUXC → 200, titled "Soroban Budget Assert — CI budget assertions & network-verified cost report demo"

• https://asciinema.org/a/qqC0RysuCDBvfUXC.svg → 200, image/svg+xml (the README badge)

• https://asciinema.org/a/qqC0RysuCDBvfUXC/iframe → 200 (the landing-page embed)

• Added/Updated tests

• Passed cargo test

• Passed cargo clippy

• Formatted with cargo fmt

• Reviewed or re-measured budget limits in amm-pool-contract/tests/budget_test.rs when contract, build profile, or toolchain changes may affect them

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update