Fix docs.rs build (crate-root feature(doc_cfg)) + add docs.rs CI job

[MagicalTux]

docs.rs failed to build compcol-0.6.4. Root cause + the CI gap that hid it, both fixed here.

Root cause

Modules across the crate carry #![cfg_attr(docsrs, doc(cfg(feature = "...")))] labels (so docs.rs shows which feature each item needs), but the crate root never enabled the nightly doc_cfg feature. docs.rs builds on nightly with --cfg docsrs, so every one of those labels errors:

error[E0658]: `#[doc(cfg)]` is experimental
   = help: add `#![feature(doc_cfg)]` to the crate attributes to enable

Why CI didn't catch it

Our existing docs CI job runs on stable without --cfg docsrs. Under those conditions the cfg_attr(docsrs, …) labels are inert, so the build is clean — the failure only appears in the docs.rs environment. (Notably feature(doc_cfg) was never present — git log -S confirms — so docs.rs builds had been fragile; recent nightly enforces it strictly.)

Fix

1. src/lib.rs: add #![cfg_attr(docsrs, feature(doc_cfg))] at the crate root — the standard companion to per-module doc(cfg(...)). It's docsrs-gated, so stable builds are completely unaffected (verified).
2. .github/workflows/ci.yml: new docsrs job that builds the docs exactly as docs.rs does — nightly + --cfg docsrs, warnings denied — so this class of failure is caught before publishing, not after.

Verification

• Before: RUSTDOCFLAGS="--cfg docsrs -D warnings" cargo +nightly doc --all-features --no-deps → E0658 (many modules).
• After: same command → clean (Generated …/index.html, no errors/warnings).
• Plain stable cargo doc --all-features → still clean; cargo build --all-features → unaffected.
• The new CI job reproduces the docs.rs build and is green on this fix.

This needs a patch release (0.6.5) to unbreak docs.rs for the published crate.

🤖 Generated with Claude Code