feat(sdk): support signing on edge and browser runtimes

[timo126]

Summary

Adds portable Ed25519 settlement signing for @accensa/sdk, enabling route-attribution reports from edge and browser-compatible runtimes while preserving Node.js support.

Changes

• Feature-detects globalThis.crypto?.subtle and attempts WebCrypto Ed25519 first.
• Falls back to node:crypto only when WebCrypto is unavailable or does not support Ed25519.
• Shares the RFC 8410 PKCS#8 wrapper for raw 32-byte Ed25519 seeds between signing paths.
• Validates private keys before constructing the PKCS#8 key.
• Keeps unsupported environments loud and actionable instead of sending unsigned reports.
• Adds coverage for unavailable signing capability and preserves existing settlement delivery coverage.
• Documents supported and tested runtimes: Node.js, Vercel Edge Functions, Cloudflare Workers, and Deno Deploy.

Verification

• pnpm --filter @accensa/sdk test — 117 tests passed.
• pnpm --filter @accensa/sdk typecheck — passed.
• git diff --check — passed.

Compatibility

Signatures remain Ed25519 signatures over the exact UTF-8 request body and continue to use the existing X-Signature header, so the /api/hook/settle verification contract is unchanged.

Closes accensa#101