Add sbrk syscall and heap management from ch4 address space design

[Copilot]

The ch4 branch's user-mode address space layout includes dynamic heap management via sbrk, but the main branch was missing the entire heap infrastructure. This adds it.

User address space layout (ch4 design)

[ELF segments] [guard page] [user stack] [heap →] ... [TrapContext] [Trampoline]

The heap starts at ustack_base + USER_STACK_SIZE, initialized as an empty MapArea, and grows/shrinks via sbrk.

Kernel changes

• mm/memory_set.rs: Add shrink_to / append_to on both MemorySet and MapArea to support page-granular heap resize
• task/process.rs: Add heap_bottom + program_brk to ProcessControlBlockInner; add change_program_brk() method; initialize empty heap MapArea in new() and exec(), copy fields in fork()
• task/mod.rs: Expose change_program_brk(size: i32) -> Option<usize> as a public API
• syscall/mod.rs: Add SYSCALL_SBRK = 214 and dispatch to sys_sbrk
• syscall/process.rs: Implement sys_sbrk

User-space changes

• user/src/syscall.rs: Add SYSCALL_SBRK + sys_sbrk
• user/src/task.rs: Add sbrk(size: i32) -> isize wrapper
• user/src/bin/sbrk_test.rs: Add test covering allocate, write, bulk grow, bulk shrink, and underflow guard

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