Add share, flatMapLatest, and reductions to the README algorithm catalog

[MUSE-CODE-SPACE]

Fixes #408, fixes #379.

The README's Contents section is the primary discovery surface for the package, but three shipped public APIs are missing from it even though each has a full DocC guide:

• share(bufferingPolicy:) — Guides/Share.md (the gap reported in both issues; users have assumed the feature doesn't exist)
• flatMapLatest(_:) — Guides/FlatMapLatest.md
• reductions(_:_:) — Guides/Reductions.md

This audits the catalog against Sources/AsyncAlgorithms/AsyncAlgorithms.docc/Guides/ (as #408 requests) and adds the three missing entries to the "Other useful asynchronous sequences" section, with one-line descriptions taken from each guide. All three are public on main (AsyncShareSequence.swift, FlatMapLatest/AsyncFlatMapLatestSequence.swift, AsyncInclusiveReductionsSequence.swift). Every other guide in the directory is already listed.

Note: a previous PR for the share entry alone (#401) was closed by its author after going unreviewed; this supersedes it and covers the full audit.

[MUSE-CODE-SPACE]

Force-pushed to amend the commit message only (removed a stray tooling attribution trailer) — the diff is unchanged. This also re-triggers CI, where the previous run hit an unrelated infra failure installing the Wasm Swift SDK (segfault on first attempt, then "bundle already installed" on retries).