docs(sdk/foundry): make T3 canonical, consolidate T2 deltas into migration appendix

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Follow-up to #350. PR #350 made T3 the canonical baseline across the protocol specs and consolidated T2-era deltas into a single ## T2 → T3 migration appendix per page. The Foundry SDK page was missed — it still has two inline #### T2 -> T3 changes headings plus several "Post-T3 / pre-T3" future-tense phrasings.

This PR applies the same pattern to src/pages/sdk/foundry/index.mdx.

Changes

• Removed both #### T2 -> T3 changes headings (under the cast interaction examples and under cast keychain).
• Rewrote surrounding prose as T3-canonical:
  • forge create note: "Post-T3, access keys can sign calls but not deployments." → "Access keys can sign calls but not deployments."
  • Access-key CREATE restriction note now reads as a current rule, not a future one.
  • cast keychain intro now describes the supported authorization options directly instead of framing them as a T3 delta.
  • Dropped "Post-T3, …" from a comment in the cast keychain code block.
• Added ## T2 → T3 migration appendix at the bottom with an :::info[Migration appendix] callout (matching the protocol specs in T3 docs cleanup: make T3 canonical, consolidate T2 deltas into migration appendix #350). The appendix contains:
  • The pre-T3 → tuple-form authorizeKey(...) ABI migration note.
  • The full ### Check your authorizeKey(...) migration with cast section, with internal anchor links updated to point back to the canonical cast send and cast keychain examples above.

Out of scope

• The migration appendix here is intentionally temporary — it should be deleted in the same future revision that drops the appendices added by T3 docs cleanup: make T3 canonical, consolidate T2 deltas into migration appendix #350, once the integrator cooldown period is over.

Verification

• grep confirms the only remaining Post-T3/pre-T3/T3 activation phrasings live inside the new ## T2 → T3 migration appendix.
• Will run corepack pnpm run check and corepack pnpm run check:types in CI.

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Project	Deployment	Actions	Updated (UTC)
tempo-docs	Ready	Preview, Comment	Apr 29, 2026 2:52pm

[max-digi]

Folded into #350 — the same diff is now commit 1444626 on mmp/remove-t3-upcoming-banner, so all T2→T3 cleanup (protocol specs + sdk/foundry) ships in one PR. Closing to avoid duplicate review.