Release 3.9.0

[MichaelGHSeg]

Bumps $SEGMENT_VERSION and dates the existing Unreleased changelog section. No code changes.

Releases the HTTP work already on master:

• Dual-path retry: exponential backoff + rate-limit handling #251 — dual-path retry: exponential backoff + rate-limit handling
• Bound the rate-limit retry budget at 5 minutes #254 — bound the rate-limit retry budget at 5 minutes

Minor rather than patch: new X-Retry-Count request header and retries that now take materially longer than the old ~13s ceiling, both carrying upgrade notes, but nothing removed from the API.

Done by hand rather than with VERSION=3.9.0 make release, deliberately. That target runs git changelog -t, which prepends a generated changelog and would bury the curated section written with the HTTP work. Its printf also rewrites lib/Version.php from scratch, dropping declare(strict_types=1); and switching to double quotes.

After merge: git tag 3.9.0 && git push origin 3.9.0 — bare tag, no v. Packagist picks it up by webhook; nothing is uploaded and there is no publish workflow.