bootc: Disallow (e)println! in crates/lib via clippy #37

Status (2026-09-23): cgwalters picked option A (explicit ProgressWriter, split into compile-checked commits). Draft fork PR updated with a 15-commit series: sync ProgressWriter, Message event (protocol 0.2.0), per-subsystem conversions, then the clippy deny. Deferred: --progress-fd for install/rollback.

Source: bootc-dev/bootc#2449 (comment)

cgwalters in review of bootc#2449: "This needs to use the progress API. We may need a blanket ban on (e)println! in crates/lib". bootc clippy.toml currently only has disallowed-methods for str::len; add disallowed-macros (or a crate-level lint) and fix/allow existing uses.