refactor(cli): drop --fallback-dir

[Maxanox]

Summary

Removes the --fallback-dir option from typst compile; the option moves out of this repository's scope.

Removed

• The --fallback-dir flag (args.rs).
• The fallback lookup in SystemFiles::resolve / load (world.rs), back to the upstream shape.
• The FallbackDirNotFound / FallbackDirNotADirectory world-creation errors and the matching retry arm in watch.rs.
• The fallback unit tests (world.rs) and integration tests (tests/cnd.rs).
• The mention in CONTEXT.md.

Kept

• --format cnd, --inputs-file, font files in --deps reported by canonical path.
• deps_report_font_files_by_their_canonical_path now exercises only --font-path (a symlinked directory outside the root, a relative path inside it).
• The hermetic environment handling of the CLI tests.

Test plan

• cargo fmt --all -- --check
• cargo clippy -p typst-cli -p typst-cnd --all-targets -- -D warnings
• cargo test -p typst-cnd, cargo test -p typst-cli
• cargo test --workspace --no-fail-fast (285 unit/integration passed, 0 failed; 3779 suite tests passed)
• git grep -n fallback crates/typst-cli: only an unrelated mention of glyph fallback remains

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