docs: specify hinted TEE registration

[leopoldjoy]

What changed

• correct the planned feature from a general TEE infrastructure migration to the Nitro signer-registration migration
• link the feature to the registrar specification
• replace the obsolete ZK/Boundless registrar description with the implemented hinted P-384 flow, including certificate caching, revocation, recovery, and upgrade compatibility

This PR is stacked on #1953.

Validation

• node scripts/validate-docs-structure.js
• node scripts/lint-mdx.js docs/specifications/base-protocol/proofs/registrar.mdx (no errors; one advisory Mermaid highlight warning)
• node scripts/lint-mdx.js docs/upgrades/upcoming-features.mdx
• npm test

Screenshots

Hinted TEE Registration

Registrar

Upcoming Features

[mindapivessa]

Could you add a preview link for this PR? I use it to review the change before approving. Thanks!

[cb-heimdall]

🟡 Heimdall Review Status

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Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1