feat(proc): added envp plumbing through proc_create

[FlareCoding]

Summary

• Programs like Vim need TERM and HOME, but the kernel wrote an empty envp block and had no way to carry environment into a child.
• proc_create gains an envp argument copied with the same bounded limits as argv, now shared constants (MAX_ARG_STRLEN, MAX_ARG_STRINGS) between the copy path and the stack builder so the two can never drift.
• libstlx proc_create passes the caller's environ through (inheritance by default, matching cwd and stdio), with proc_create_with_env for explicit control. The kernel never invents environment values.

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