perf: avoid recursive dir-size calculation in non-size views

[Maanas-Verma]

Summary

• Fix performance issue from slow on big directories ( macos ) #152 where lla -g is slow in large parent directories when include_dirs=true.
• Avoid recursive directory size aggregation in views that do not consume size metadata (grid/default/tree/timeline/git/recursive human output).
• Preserve recursive directory sizing for size-aware paths: long/table/sizemap/fuzzy, sort-by-size, size filters, and machine-readable outputs.
• Clarify include-dirs and config docs as recursive and potentially expensive.

Root Cause

When include_dirs=true, list_and_decorate_files computed recursive size for every directory entry unconditionally. In large directory trees, this dominated runtime/system time even for grid output where size is not rendered.

Validation

• cargo fmt
• cargo test -p lla
• Manual benchmark comparison:
  • Grid view remains fast with include_dirs=true.
  • Long view with include-dirs remains expensive as expected for size-aware output.

Attribution

This change was implemented via Codex.

[Maanas-Verma]

@chaqchase, I have tested it out and it's works fine. Can you please test and merge it from your end.