Removes base-index detection which was unreliable and defaulted to 0
when tmux's actual base-index was 1, causing "index in use" and
"can't find window" errors on session creation.
cache.rs: make minimal_roots pub(crate); add 8 tests covering the
minimal_roots helper (empty input, single root, nested vs sibling dirs)
and validate_and_update (stale project removal, no-change short-circuit,
mtime-triggered rescan, legacy empty dir_mtimes).
session.rs: make session_name pub(crate); add 5 tests covering session
name sanitisation (dots→underscores, spaces→dashes, fallback for root
path) and TOML parsing for Window and SessionConfig with layout.
config.rs: add 3 tests covering serde defaults when optional fields are
absent, full config parsing and invalid TOML rejection.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
