AOT: Link multiple translation units into one binary

[PurHur]

Problem

bin/compile.php emits one .o per entry file. Multi-file apps need linking multiple objects + runtime.

Goal

phpc build produces single executable from entry + included modules (#59).

Tasks

• Track symbols across compilation units in project build
• Extend lib/AOT/Linker.php for multiple .o
• Resolve duplicate superglobal init
• Test: entry includes helper.php

Acceptance

Two-file app links and runs as one binary.