[PHP] fix: split fork/process/shared-memory runtime fixes

[brandonpayton]

Summary

• split the remaining fork/process/shared-memory runtime fixes out of mixed PHP PHPT PR Preserve shared mappings across fork for PHP PHPTs #717
• stack on [PHP] fix: split socket datagram and multicast semantics #734 via Automattic branch integration/kd-6nz-php-phpt-socket-network-base because the FPM socket/shared-backlog commit depends on the socket split
• keep this PR targeted at the PHP harness stack, not main

Stack

• Harness PR: [PHP] Add upstream PHPT harness #736
• Direct base: integration/kd-6nz-php-phpt-socket-network-base (#736 harness + #734 socket/network)
• Depends on [PHP] fix: split socket datagram and multicast semantics #734: [PHP] fix: split socket datagram and multicast semantics

Preserved commits

• 6ecec96 fix: isolate PHPT workers and preserve process status
• 3d27d6e fix: share anonymous mmap across fork
• 74f7375 fix: honor mmap address hints
• 940d885 fix: improve shared memory and fpm socket semantics
• 5368b79 fix: restore fork socket exec reader
• 8496bd0 fix: support fork from dlopened wasm side modules
• fb768e0 Preserve shared mappings across fork for PHP PHPTs
• 9a7ec18 host: avoid single-observer shared-memory sync churn

Verification

• git diff --check origin/integration/kd-6nz-php-phpt-socket-network-base...HEAD

[brandonpayton]

Closing as superseded by origin-headed replacement #756: #756

This PR head was in brandonpayton/kandelo-agentic-dev. The replacement uses the same head SHA (9a7ec18) on an Automattic/kandelo origin branch and has an expanded description explaining why the change is needed and why it belongs in Kandelo.