lib.rs publishes 17 uncurated pub modules — declare the library surface unstable

[FelixKrueger]

Raised by #399's dual plan review (plans/08082026_normpath-visibility/PLAN_REVIEW_A.md, Optional-2).

src/lib.rs has 17 bare pub mod lines and no pub use curation, so every internal helper in the crate is published API. The crate is published (release.yml:421-441 crates.io job; no publish = false, no [lib] restriction), so docs.rs renders trim_galore::io::* and any future visibility tightening — starting with #399's norm_path demotion — is technically a semver-breaking API removal from a 2.x crate.

Nothing plausible depends on TrimGalore as a library; the lib target exists so unit tests can reach internals. But that intent is nowhere stated, so each demotion re-raises the question.

Fix: one //! note in src/lib.rs declaring the library surface an implementation detail of the binary with no semver guarantee. That pre-authorises #399 and its successors, and costs nothing.

[FelixKrueger]

Fixed by #411 (2e00f32). src/lib.rs now carries a module doc stating that the library target exists so the binary and integration tests can reach internals, that its surface is an implementation detail, and that it carries no semver guarantee. This pre-authorises visibility tightening like #399's norm_path demotion instead of re-raising the question each time. Closing manually.