feat: Reduce ipsw diff memory usage

[PoomSmart]

Problem

ipsw diff consumed 48GB+ RAM, crashing on 16GB and lower memory machines.

Changes

This PR contains improvements for in-memory diffing and the addition of disk-cached diffing.

1. Streaming DMG Traversal (internal/search/search.go)

• File paths are now streamed directly to handlers instead of collecting into []string slices
• Eliminates gigabytes of path string accumulation for large DMGs

2. Per-Image DSC Diff (internal/commands/dsc/diff.go)

• Diff images one-at-a-time instead of building two huge map[string]*DiffInfo maps
• Close MachO handles immediately after comparison

3. Optional Disk Caching (internal/commands/macho/diff.go)

• Added --low-memory flag for low memory machines
• Default: fast in-memory mode (original behavior)
• Low-memory: caches old IPSW's DiffInfo to temp gob files, loads on-demand

4. Stream gob.Encode (internal/diff/diff.go)

• Save() now streams directly to file instead of buffering in bytes.Buffer

Usage

# Default (fast, needs large amount of RAM)
ipsw diff old.ipsw new.ipsw -o out -m

# Low-memory mode (slower, works on low memory machine)
ipsw diff old.ipsw new.ipsw -o out -m --low-memory

Fixes #985

[blacktop]

Have you done a speed comparison between low-memory and normal mode? or are you not able to due to the limitations of your host?

[PoomSmart]

@blacktop Yeah unfortunately I cannot perform normal mode test. My machine just doesn't have enough RAM.