Memory management: per-process VA range, brk frontier tracking, noreserve/LOW_2G support

[wdcui]

Summary

Extends the memory management subsystem to support per-process virtual address ranges and adds several new capabilities needed by the host process sandbox.

Stack: PR 6 of N — depends on #745

Changes

Per-process VA range (Vmem, PageManager)

• Vmem::new() and PageManager::new() accept a Range<usize> instead of always using the platform-wide TASK_ADDR_MIN..TASK_ADDR_MAX
• All address validation (insert_mapping, get_unmmaped_area, handle_page_fault) uses the configured range instead of platform constants
• Reserved ranges from the platform are clamped to the configured range
• Cached addr_min/addr_max on PageManager for lock-free bounds checks

Heap (brk) improvements

• brk_base: floor that prevents shrinking below the initial program break
• brk_frontier: page-aligned high-water mark that tracks the actual backed extent, replacing the old brk.next_multiple_of(PAGE_SIZE) computation
• ensure_brk_past(): advances the floor/brk/frontier past non-heap mappings (e.g., trampolines)
• current_brk() / current_brk_frontier(): read-only accessors

New CreatePagesFlags

• FD_WRITABLE (1<<7): shared file-backed mapping gets VM_MAYWRITE when the fd is writable
• NORESERVE (1<<8): sparse reservation without swap/commit upfront
• LOW_2G (1<<9): constrain mapping below the MAP_32BIT 2 GiB ceiling
• Backing type widened from u8 to u16

Allocation robustness

• create_mapping retry loop: non-fixed allocations that fail because the host rejected the address fall back to the next candidate VA instead of returning ENOMEM immediately
• insert_mapping validates the platform-returned address is within the managed range; frees stray allocations
• get_unmmaped_area supports require_low_2g and max_start narrowing for the retry loop
• VmemResizeError::OutOfRange triggers move_mappings fallback in remap_pages

Miscellaneous

• VmArea.noreserve field, propagated through all split/merge/copy paths
• may_flags_for_mapping considers fd_writable for VM_MAYWRITE
• handle_page_fault returns AllocationFailed instead of panicking
• VmemPageFaultHandler::HANDLE_USER_PAGE_FAULTS const (default false)

Files changed (8 files, +415/-96)

File	Change
litebox/src/mm/linux.rs	Core Vmem changes: VA range, VmArea noreserve, CreatePagesFlags, retry loop, get_unmmaped_area
litebox/src/mm/mod.rs	PageManager: VA range, brk_base/frontier, handle_page_fault, remap OutOfRange
litebox/src/mm/tests.rs	Updated Vmem::new and insert_mapping call signatures
litebox_shim_linux/src/lib.rs	PageManager::new caller with full platform range
litebox_shim_optee/src/lib.rs	PageManager::new caller with full platform range
litebox_shim_linux/src/syscalls/mm.rs	register_existing_mapping fd_writable param
litebox_platform_linux_kernel/src/mm/tests.rs	Fully-qualified PageManager::new range param
litebox_platform_lvbs/src/mm/tests.rs	Fully-qualified PageManager::new range param

Testing

• cargo build — clean
• cargo clippy --all-targets --all-features — clean
• cargo fmt --all -- --check — clean
• cargo nextest run --profile ci -E 'not test(nine_p)' — 208 passed, 15 failed (pre-existing TUN/EPERM), 34 skipped