feat: add PHP-compatible PCNTL support

[nahime0]

Summary

• add PCNTL support across the builtin contract, EIR lowering, target-aware code generation, runtime helpers, and the optional elephc-pcntl bridge
• implement process control with pcntl_fork(), wait/waitpid status handling, priorities, process groups and sessions, daemonization, and exec
• implement PHP-compatible signal registration, delivery, dispatch, masking, alarms, async signals, signal information, and synchronous signal waiting
• provide PCNTL support in Magician, including callable handlers, by-reference outputs, restart-syscall behavior, and exception-safe dispatch
• preserve PHP semantics for forked signal queues, failed waits, dispatch snapshots, reentrancy, exceptions, invalid signals, OS warnings, and Fiber switching during handlers
• support macOS AArch64, Linux AArch64, and Linux x86_64 through target-specific constants and runtime behavior
• add focused bridge modules, end-to-end and error regressions, a PCNTL user guide, --with-pcntl documentation, generated compatibility data, and the completed implementation checklist

Testing

• cargo build
• cargo test -p elephc-pcntl --lib
• cargo test -p elephc-magician omitted_restart_syscalls_is_false_only_for_sigalrm
• cargo test --test codegen_tests pcntl — 34 passed on macOS AArch64
• ./scripts/test-linux-x86_64.sh pcntl — 35 PCNTL codegen tests and 17 PCNTL error tests passed
• ./scripts/test-linux-arm64.sh pcntl — 35 PCNTL codegen tests and 17 PCNTL error tests passed; the wrapper subsequently exited with code 101 because Cargo attempted to execute a missing, zero-match Magician test binary
• cargo build --example gen_builtins
• python3 scripts/docs/extract_builtins.py --render --force
• python3 scripts/docs/test_gen_php_comparison.py
• python3 scripts/docs/gen_php_comparison.py
• python3 scripts/docs/audit_builtins.py
• python3 scripts/docs/elephc_builtins/validate_site_compat.py
• python3 scripts/audit_builtin_eir_boundary.py --enforce-target-architecture
• assembly comment checks for the touched emitters
• cargo fmt --all -- --check
• git diff --check