YNU-864: Learn introduction rewrite (v1 mental model)

[ihsraham]

Summary

Rewrites the most-read Learn introduction pages around the native v1 model: ChannelHub, Nitronode, signed channel states, home/non-home ledgers, escrow, commit/release app-session funding, and the void/open/challenged/closed lifecycle.

What changed

• Updated the Learn index to describe Nitronode, split YELLOW-token utility from free/open-source SDK tooling, and point 0.5.3 readers at the planned migration page.
• Rewrote what-yellow-solves to teach state channels with v1 channel-state and ChannelHub enforcement terminology.
• Rebuilt architecture-at-a-glance around application, SDK/v1 RPC, Nitronode, and ChannelHub surfaces, including the CHALLENGED lifecycle state.
• Replaced stale supported-chain contract address tables with runtime discovery through client.getConfig() / node.v1.get_config and client.getAssets() / node.v1.get_assets.
• Updated the Petal Diagram page so app-session funds are described as commit/release extension states, not Node-wallet custody.

Source of truth

Architectural claims are sourced from nl/docs/protocol/*.md, the existing yellow-docs/docs/nitrolite/protocol/*.mdx pages, nl/contracts/src/ChannelHub.sol, nl/contracts/deployments/, nl/MIGRATION-NITRONODE.md, nl/docs/api.yaml, and the native TypeScript SDK getConfig / getAssets implementations.

Resolves: F-010, F-011, F-012, F-036, F-037.

Validation

• Vale on the five touched files: 0 findings.
• PR diff Vale advisory lint: 0 findings.
• Source verification grep checks pass for removal of Custody/Adjudicator/VirtualApp/Nitro RPC Protocol, legacy hardcoded addresses, DISPUTE, and stale Node custody language.
• ChannelHub, Nitronode, CHALLENGED, getConfig, get_config, and github.com/layer-3/nitrolite checks pass.
• Five edited pages total 494 lines, under the reviewability target.
• npm run build succeeds with the same pre-existing Docusaurus deprecation, 0.5.x, and Clearnet broken-link/anchor warnings only.
• Local static server smoke check: /nitrolite/learn/introduction/architecture-at-a-glance serves successfully from the built site.

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