fix(core): deprecated interpolation, var_dump variadic, print_r $return, array ref alias, Windows PE stub

[Guikingone]

Closes #340, #389, #331, #379.

#340: String interpolation ${var} (deprecated)

• Added ${expr} form to the lexer interpolation routine
• Re-lexes the braced expression (with leading $ prepended) as a normal PHP expression
• PHP 8.x deprecates this syntax but still supports it

#389: var_dump variadic + print_r $return mode

• var_dump: signature changed to variadic; each operand dumped independently in source order
• print_r: signature accepts optional $return bool; when true, captures output to a buffer
• New globals _print_r_mode/_print_r_off/_print_r_buf redirect __rt_stdout_write and walker writes
• New runtime helpers __rt_pr_write, __rt_pr_append, __rt_pr_finish (both AArch64 + x86_64)
• print_r without $return now returns PHP true instead of void

#331: Reference alias to array elements (source-side)

• $b =& $a[0] creates a reference alias from a local to an array element
• New Op::LoadArrayElemRefCell (2 operands: array + index)
• Type checker ArrayAccess arm in check_ref_assign
• EIR lowering via bind_local_ref_cell_ptr (inline-storage pointer approach)
• Codegen for AArch64 + x86_64
• Indexed arrays only; hash arrays and target-side ($a[0] =& $b) are follow-up

#379: Windows PE stub

• Added Platform::Windows to the enum with detect_host, php_os_name, extern_symbol, etc.
• All Platform/Arch match arms now have a Windows arm that panics with a clear message
• Parse windows-x86_64 / x86_64-pc-windows-msvc / x86_64-pc-windows-gnu target strings
• supports_current_backend() returns false for Windows
• Full Windows implementation (MSx64 ABI, PE/COFF, Win32 calls) is staged for follow-up PRs

Test results

• cargo build: 0 errors, 0 warnings
• printing tests: 46 passed
• references tests: 23 passed
• var_dump tests: 23 passed
• interpolation tests: 45 passed
• lexer interpolation tests: 7 passed