ci: add auto-updating test coverage badge to README

[artall64]

Summary

Adds an auto-updating test coverage badge to the README, computed entirely inside GitHub Actions (no third-party services). Same approach as 1inch/cross-chain#428.

• README: embeds the badge at the top, served from the orphan badges branch via https://github.com/1inch/cross-chain-sdk/raw/badges/coverage-badge.svg.
• .github/workflows/coverage-badge.yml: on every push to master (and manual dispatch) it installs dependencies, runs pnpm test:cov, renders the badge SVG, and force-pushes it to the badges branch. On PRs it runs only when the coverage tooling itself changes, and skips publishing. Runner/pnpm/node setup mirrors pr-check.yml (ubuntu-latest, pnpm 10, node 20).
• scripts/generate-coverage-badge.mjs: renders a shields.io-style SVG (total line coverage, color-coded) from Jest's json-summary reporter output.
• package.json: adds a test:cov script (jest --coverage with text-summary + json-summary reporters).
• jest.config.js: coverage reports now land in <repo>/coverage (gitignored) instead of src/coverage — coverageDirectory is resolved relative to rootDir (src), so the previous value wrote inside the source tree.

The badges branch has been seeded with the real current value (88.2% line coverage), so the README renders immediately; the workflow refreshes it on every subsequent push to master.

Verification

• The pull_request run of the Coverage badge workflow on this PR passed (23 suites / 133 tests, total line coverage 88.2%).
• The PR validation failure is unrelated: npm retired its legacy audit endpoint (HTTP 410 from registry.npmjs.org/-/npm/v1/security/audits), which breaks the pnpm audit step on every branch as of today — the same step passed on other PRs as recently as July 14.

Notes

• Coverage reflects the unit test suite (jest.config.js); integration tests are not included.
• The publish step relies on the workflow GITHUB_TOKEN having contents: write (declared in the workflow). If the org enforces read-only workflow tokens, the push step will fail and a PAT/deploy key would be needed instead.