Add incremental strict TypeScript ratchet

[raymondidahor-bot]

Summary

Closes #

How was this tested?

Merge requirements

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• All required CI checks pass on the latest commit (not just an earlier push).
• No required checks are failing, pending, or skipped — re-run or fix them; do not ask for a merge while any are outstanding.
• The branch has no merge conflicts with the target branch (rebase or merge master if GitHub shows "This branch has conflicts").
• Tests were added or updated for the change (primary flow, a boundary case, and a failure case).
• Docs were updated where behaviour, configuration, or security posture changed.

Security-sensitive changes

• This PR touches a path covered by .github/CODEOWNERS (wallet, auth, cryptography, CI) and a code owner has been requested for review.
• I described any change to key handling, signing, session lifetime, or trusted endpoints above.

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Close #960

Summary

• Add tsconfig.strict.json for the incremental strict TypeScript migration surface.
• Add a ratchet check that rejects removal of existing approved entries.
• Add CI and package scripts for the strict type check and allowlist validation.
• Add tests for the baseline, an allowlist addition, and an attempted removal.
• Document the contributor workflow and update the targeted Stellar read-source typing.

Validation

• node scripts/check-strict-tsconfig.mjs passes.
• pnpm exec vitest run tests/unit/strictTypeConfig.test.js --config vitest.config.js passes (3 tests).

Known issue

npx tsc -p tsconfig.strict.json --noEmit currently fails with TypeScript diagnostics, including errors in files pulled into the compilation through dependencies. The strict type-check CI gate is therefore not yet confirmed green and should be resolved before merge.

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