[kernel] Add optional GEM INT EF resident broker

[parabyte]

Summary

This adds a default-off, real-mode IBM PC broker for the original GEM INT EFh AES/VDI entry point.

• installs the signed GEMAES vector only while one resident user process is registered
• transfers requests through one fixed slot per ELKS task
• pins the client's native data segment while the resident process handles the request
• reports exec and exit lifecycle records so retained process state is released safely
• keeps AES, VDI, menus, windows, and desktop policy in user space

The control call uses syscall slot 65, whose former module-loading operation is already removed.

Compatibility and bounds

CONFIG_GEM_TRAP defaults to off and is offered only for IBM PC real-mode builds. The direct IVT path is excluded when CONFIG_286_PMODE is selected.

All public fields, registers, limits, counters, and generation halves are 8 or 16 bits. The broker uses a fixed 640-byte table, performs no dynamic allocation, and preserves far addresses as explicit segment and offset words.

The saved vector is restored only if INT EFh still points to this broker. Owner exec, owner exit, client exec, client exit, interruption, cancellation, and stale replies all have explicit cleanup paths.

Validation

Validated at commit e1cf24a810c7c1e74d1ca53f84a8a8ca66a2d432 on upstream master b7cfe4973487ab235169dd9eb489285409fb8b2a with an IBM PC configuration generated with CONFIG_GEM_TRAP=y:

• full kernel build: pass
• gemtrap.o and broker test-object 8086 instruction audits: pass
• source and executable-model gates: pass
• headless QEMU isapc guest regression: GEM_TRAP_BROKER_PASS
• disposable test image verified; source image remained unchanged
• resulting kernel: 116208 bytes total, including 64096 bytes near code and 32832 bytes far text

This is deliberately separate from the GEM user-space desktop integration so the kernel ABI can be reviewed on its own.

Related background: #871.