[kernel] Fix medium-model signal handler range

[parabyte]

Summary

This fixes sys_signal() validation for medium-model executables whose combined code allocation crosses 64 KiB.

The current check converts the allocation from paragraphs to bytes with s->size << 4. That 16-bit expression wraps at 64 KiB, and requiring the handler segment to equal the allocation base also rejects valid far-text handlers.

The replacement:

• keeps the complete check in native 16-bit paragraph units
• accepts a handler segment within the process code allocation
• accounts for complete paragraphs in the 16-bit handler offset
• orders subtraction and bounds checks so no intermediate can wrap
• retains the existing default and ignore signal behavior

The byte offset is converted with four single-bit 8086 shifts, avoiding a division helper or variable-count shift.

Validation

Validated at commit 548d73db1fc57f4f11c7e7379f87461077ca23eb on upstream master b7cfe4973487ab235169dd9eb489285409fb8b2a:

• full IBM PC ibmpc-1440 kernel build: pass
• resulting kernel: 113056 bytes, including 62352 bytes near code and 32112 bytes far text
• changed sys_signal disassembly: 16-bit 8086 instructions only
• no multiply, divide, wide helper, or later-CPU opcode in the changed routine
• patch whitespace and one-commit branch audit: pass

Related signal audit discussions: #2646 and #2649.