fix(types): enable noUncheckedIndexedAccess

[andrei-hasna]

Summary

• enable noUncheckedIndexedAccess in the root TypeScript configuration
• handle all 28 resulting diagnostics with iteration, validation, explicit guards, or safe test access
• document the stricter typechecking in the Unreleased changelog

Verification

• npx --yes bun install
• npx --yes bun run typecheck
• npx --yes bun test (1,053 passed, 0 failed)

Task validation

Freshly fetched origin/main at fafb395 on 2026-07-29 still had strict: true without noUncheckedIndexedAccess, so the task was not stale.

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