docs(readme): add bilingual DDN discoverability

[mikko-lab]

Summary

• Sharpens README.md's opening for an international software audience: "Open-source reference architecture for cryptographically verifiable decisions executed by deterministic, versioned business rules." Restructures it under GEO-friendly headings — What DDN is, Who it is for, What a decision receipt proves, What DDN does not prove, Reference architecture, Current evidence and limitations, Getting started, Security. Uses "cryptographically verifiable decisions," "deterministic policy execution," "auditable decision receipts," "policy-as-code," "automated decision systems," and "independent receipt verification" naturally in prose, not as a keyword list.
• Adds README.fi.md, a content-faithful Finnish parallel with the same heading structure, same technical terms, same scope limits, and the same links — no softened claims.
• Both files open with reciprocal language links (**English** | [Suomi](README.fi.md) / [English](README.md) | **Suomi**).
• All existing technical limitations from docs/trust-model.md, docs/limitations.md, docs/public-project-description.md, SECURITY.md and CONTRIBUTING.md are preserved verbatim in substance: reference implementation (not a production validator network), fixed 2-of-3 trusted set (not permissionless/BFT), same-host/same-binary validator processes, EVM anchoring demonstrated only on local Anvil, in-memory state, no KMS/HSM or independent operators, no claim that DDN proves a business rule correct/fair/lawful, no claim that DDN verifies AI/LLM reasoning (only that its deterministic policy layer can serve as an audit boundary alongside such a system).
• node scripts/public-release-check.mjs --mode git-worktree → PASS (436 allowlisted files).
• No source code, .github/workflows/ci.yml, or GitHub repository metadata (description/homepage/topics) changed by this PR.
• Homepage is intentionally not proposed/set anywhere here — no confirmed, public, permanent DDN Explorer URL exists in the repository.
• A follow-up GitHub metadata change (description + 15 topics) is planned separately, outside this PR, using the reproducible-builds topic in place of verifiable-computing (chosen to avoid implying ZK/SNARK-style verifiable computation that this reference does not implement).

Test plan

• git diff --check / git diff --cached --check — clean, no whitespace errors
• Reciprocal language-link check between README.md and README.fi.md
• All relative Markdown links in both files resolve
• Heading structure and key technical-term mention counts match 1:1 between README.md and README.fi.md
• .github/workflows/ci.yml and source code confirmed unchanged
• node scripts/public-release-check.mjs --mode git-worktree → PASS (436 allowlisted files)