[PHP] fix: split TCP close semantics stack

[brandonpayton]

Summary

• split the remaining TCP close semantics fixes out of PHP PHPT PR Preserve shared mappings across fork for PHP PHPTs #717
• stack on the existing [PHP] fix: raise SIGPIPE for bridged TCP EPIPE #729 EPIPE/SIGPIPE split via Automattic base branch integration/kd-6nz-php-phpt-tcp-epipe-sigpipe-base
• keep this PR diff to the two TCP close commits; [PHP] fix: raise SIGPIPE for bridged TCP EPIPE #729 now targets integration/kd-6nz-php-phpt-harness-only-base, so this stack feeds the [PHP] Add upstream PHPT harness #736 harness base rather than main

Stack

• Depends on [PHP] fix: raise SIGPIPE for bridged TCP EPIPE #729: [PHP] fix: raise SIGPIPE for bridged TCP EPIPE
• [PHP] fix: raise SIGPIPE for bridged TCP EPIPE #729 base: integration/kd-6nz-php-phpt-harness-only-base
• Stack base: integration/kd-6nz-php-phpt-tcp-epipe-sigpipe-base at 9e252cf

Preserved commits

• 82dd98f fix: close bridged TCP sockets gracefully
• 3da3a8f fix: model TCP close as orderly FIN

Verification

• scripts/dev-shell.sh bash -lc 'target=$(rustc -vV | sed -n "s/^host: //p"); cargo test -p kandelo --target "$target" --lib' (962 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/tcp-backend.test.ts test/virtual-network.test.ts' (13 passed)

[brandonpayton]

Closing as superseded by origin-headed replacement #755: #755

This PR head was in brandonpayton/kandelo-agentic-dev. The replacement uses the same head SHA (3da3a8f) on an Automattic/kandelo origin branch and has an expanded description explaining why the change is needed and why it belongs in Kandelo.