[SebastianBoehler]

Prepares v4.0.0 for the changes merged since v3.0.0: trading approvals, environments, rate-limit handling, order-flow helpers, pagination, typed Gamma and Data API clients, and RTDS/Sports streams.

Updates the CMake version, public version macros, README installation examples and feature list, migration guide, and curated release notes. The major version reflects the rejection of non-137 chain IDs by string-based ClobClient constructors. The migration guide also covers consumer rebuilds, CMake major-version requirements, read retries, and settlement hash reconciliation. Release notes distinguish contributor-reported live observations from fixture coverage.

Validation on macOS arm64:

• Release build with examples and tests passed.
• All 84 offline tests passed, including test_package_consumer and both fuzz corpus replays.
• A separate consumer found the installed package as 4.0.0 EXACT, linked the new clients and streams, and checked chain-80002 rejection without network requests.
• The migration example compiled.
• Changed-file formatting checks, version-header clang-tidy, quality-script unit tests, and git diff --check passed.

Linux/macOS Debug/Release CI must pass on this commit before merging. Tag creation and release publication are separate steps; this PR does not publish a release.