[PHP] fix: split socket datagram and multicast semantics

[brandonpayton]

Summary

• split the independent socket/datagram/multicast fixes out of PHP PHPT PR Preserve shared mappings across fork for PHP PHPTs #717
• preserve the original logical commits on a branch that targets integration/kd-6nz-php-phpt-harness-only-base, the Automattic mirror of [PHP] Add upstream PHPT harness #736's harness-only head
• exclude unrelated parent-branch context already represented by other split PRs, including file-size-limit writes, Unix listener sharing, TCP graceful close, and route-netlink enum wiring

Preserved commits

• daf06b7 fix: correct socket semantics for PHP streams
• a4d1c70 fix: deliver local datagrams for PHP network tests
• 61446f9 fix: deliver IPv4 multicast loopback datagrams
• 6620c52 fix: align multicast datagram ABI support
• 0c70b7c fix: return SO_BINDTODEVICE as a string option
• f41e20d fix: reserve IPv4 ports for dual-stack IPv6 listeners

Verification

• scripts/dev-shell.sh bash -lc 'target=$(rustc -vV | sed -n "s/^host: //p"); cargo test -p kandelo --target "$target" --lib' (970 passed)
• scripts/dev-shell.sh bash -lc 'WASM_POSIX_SYSROOT=/Users/brandon/src/kandelo/sysroot cd host && WASM_POSIX_SYSROOT=/Users/brandon/src/kandelo/sysroot npx vitest run test/fetch-backend.test.ts test/virtual-network.test.ts' (26 passed)

[brandonpayton]

Closing as superseded by origin-headed replacement #754: #754

This PR head was in brandonpayton/kandelo-agentic-dev. The replacement uses the same head SHA (f41e20d) on an Automattic/kandelo origin branch and has an expanded description explaining why the change is needed and why it belongs in Kandelo.