Introduces C-side interned string registry (frankenphp_strings) and a frankenphp_server_vars struct to bulk-register known $_SERVER variables with pre-sized hashtable capacity.
This PR uses `zend_array_dup` to simplify and optimize the environment sandboxing
logic. It also guarantees no environment leakage on FrankenPHP restarts.
This PR:
- moves state.go to its own module
- moves the phpheaders test the phpheaders module
- simplifies backoff.go
- makes the backoff error instead of panic (so it can be tested)
- removes some unused C structs
