list: show fallback recipes when fallback is enabled

[veeceey]

When set fallback is active, just --list now walks up the directory tree and also displays recipes from parent justfiles. Each parent's recipes appear under a clearly labeled heading:

Available recipes:
    child-recipe # A child recipe

# Fallback recipes from /path/to/parent
Available recipes:
    build         # Another one
    parent-recipe # A parent recipe

It keeps walking as long as each parent also has set fallback enabled, and respects the same search config constraints as the run fallback logic (only works with directory-based search, not --justfile).

Changes:

• Search now derives Clone so we can iterate parent directories
• list() takes loader and search parameters to compile parent justfiles
• Added integration test

Manual test results:

$ cd /tmp/just-test/sub && just --list
Available recipes:
    child-recipe # A child recipe

# Fallback recipes from /private/tmp/just-test
Available recipes:
    build         # Another one
    parent-recipe # A parent recipe

All existing fallback (14) and list (45) tests pass.

Fixes #1500