docs: add evolution & scaling concept page (Batch 4)

[marc0olo]

Summary

• Creates docs/concepts/evolution-scaling.md with four sections: fault tolerance and node recovery (CUPs, state sync, subnet recovery), subnet creation process (NNS proposal flow, XNet messaging), and chain evolution (epoch-boundary protocol upgrades, CUP handoff)
• Updates docs/concepts/chain-key-cryptography.md: replaces Learn Hub chain-evolution link with internal path
• Updates docs/references/glossary.md: replaces Learn Hub fault-tolerance link with internal path

Staging files deleted

• .migration/learn-hub/how-does-icp-work/evolution-scaling/evolution-scaling.md → intro section
• .migration/learn-hub/how-does-icp-work/evolution-scaling/fault-tolerance.md → docs/concepts/evolution-scaling.md#fault-tolerance
• .migration/learn-hub/how-does-icp-work/evolution-scaling/subnet-creation.md → docs/concepts/evolution-scaling.md#subnet-creation
• .migration/learn-hub/how-does-icp-work/evolution-scaling/chain-evolution.md → docs/concepts/evolution-scaling.md#chain-evolution

Sync recommendation

hand-written

[marc0olo]

Review notes

Two broken links need to be resolved before merge:

Broken links (blocking, dependency on #209): The "Further reading" section links to protocol/state-synchronization.md and protocol/index.md. Neither path exists on main until PR #209 merges. These will produce dead links or build failures.

Options:

• Remove both links and add them back after docs: add protocol stack concept pages (Batch 1) #209 merges
• Batch-merge after docs: add protocol stack concept pages (Batch 1) #209

No other issues: frontmatter complete, chain-key-cryptography.md and glossary.md cross-links correctly updated, <!-- Upstream: --> comment present, no banned patterns.

[marc0olo]

Two issues to fix in docs/concepts/evolution-scaling.md before merging:

1. Em-dashes in "Further reading" bullets (lines 74–76)

Em-dashes (—) are banned project-wide per AGENTS.md. All three bullets in the "Further reading" section use them. Replace each — with : .

2. Broken links in "Further reading" (lines 74–75)

protocol/index.md and protocol/state-synchronization.md are referenced but don't exist yet — they land in PR #209. Options:

• Remove the two links temporarily and restore them after docs: add protocol stack concept pages (Batch 1) #209 merges, or
• Batch-merge this PR after docs: add protocol stack concept pages (Batch 1) #209 so the targets exist at merge time.

Either way, npm run build will fail with broken-link errors until #209 is in.

[marc0olo]

Feedback addressed:

• Replaced em-dashes with colons in all Further reading bullets
• Removed the two broken links to protocol/index.md and protocol/state-synchronization.md (both pages land in PR docs: add protocol stack concept pages (Batch 1) #209 and don't exist yet on main). The links can be restored once docs: add protocol stack concept pages (Batch 1) #209 merges. The body text reference on line 12 was also de-linked for the same reason.