chore(main): release 9.18.0

[github-actions]

🤖 I have created a release beep boop

9.18.0 (2026-09-23)

Features

• Add retry state for RETRY-spec backoff (#522) (e734558)
• warn that payload filtering has no effect with FDv2 (#518) (296311f)
• Warn when the SDK is used in a forked process without postfork (#512) (75402e9)

Bug Fixes

• Add async modules to the readthedocs API reference (#513) (f40d3b2)
• Drive repeating tasks from a delay source (9abe8cb)
• Log the cached-data evaluation warning only once per client (#520) (59ce998)
• Read all items when a collection is empty (#524) (1087147)
• Repeating task wait duration is no longer reduced by a slow callback duration. (9abe8cb)
• Report a distinct User-Agent for the async client (#516) (514c467)

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Note

Overview
This PR bumps the SDK from 9.17.0 to 9.18.0 across the release manifest, pyproject.toml, ldclient/version.py, and the provenance verification example in PROVENANCE.md. It adds a 9.18.0 section to CHANGELOG.md that documents what ships in this release (no application code changes appear in the diff itself).

The changelog highlights RETRY-spec backoff retry state, warnings when payload filtering is ineffective with FDv2 or when the client is used in a forked process without postfork(), plus fixes for repeating-task timing (delay-driven scheduling), empty collection reads, one-time cached-data evaluation warnings, a distinct async User-Agent, and Read the Docs coverage for async modules.

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