docs(ldm): align staleness check wording + scope docs.rs feature set

Three doc/metadata corrections across the LDM surface:

1. `bt::BtMatcher::prepare_ldm_candidates` (bt/mod.rs:354) and
   `ldm::LdmProducer::generate_into` (ldm/mod.rs:167, plus the
   regression-test doc at ldm/mod.rs:589) all described the
   staleness check as `entry.offset <= history_abs_start`. The
   actual implementation since commit e1932b2 is an inclusive
   lower bound: `find_best_match` rejects entries with
   `match_abs < lowest_index_abs` so entries at exactly
   `lowest_index_abs == history_abs_start` survive. Updated
   the three doc sites to describe the inclusive filter,
   pointing at `ldm::search::FindBestMatchInputs::lowest_index_abs`
   for the canonical definition.

2. `[package.metadata.docs.rs] all-features = true` would also
   enable `rustc-dep-of-std` (libstd-build-only — swaps in
   `rustc-std-workspace-*` core/alloc), `bench_internals`
   (widens the public API for benches), and `fuzz_exports`
   (widens it for fuzz targets). None of those should appear
   on docs.rs. Replaced with explicit `features = ["std",
   "hash", "dict_builder"]` so the published documentation
   covers exactly the public surface.

472 / 472 lib tests pass under default features AND
`--no-default-features`. 11 / 11 doctests pass. `cargo doc
--lib --features dict_builder,std,hash` builds without
warnings. `cargo clippy --lib --tests -- -D warnings` clean
in both feature configs.

Author: polaz

zstd/Cargo.toml

@@ -17,12 +17,18 @@ readme = "README.md"
 keywords = ["zstd", "zstandard", "decompression", "compression", "pure-rust"]
 categories = ["compression"]
 
-# docs.rs builds the crate with all features enabled so feature-gated items
-# (e.g. the `dictionary` module behind `dict_builder`) appear in the published
-# documentation. The `--cfg docsrs` flag activates the `#[cfg_attr(docsrs,
-# doc(cfg(...)))]` annotations that render feature badges on each item.
+# docs.rs builds the crate with the public feature set so feature-gated
+# items (e.g. the `dictionary` module behind `dict_builder`) appear in the
+# published documentation. We list the public features explicitly rather
+# than using `all-features = true` because the manifest also exposes
+# `rustc-dep-of-std` (libstd-build-only — swaps in `rustc-std-workspace-*`
+# crates), `bench_internals` (widens the API surface for benches), and
+# `fuzz_exports` (widens it for fuzz targets); none of these should appear
+# on docs.rs. The `--cfg docsrs` flag activates the
+# `#[cfg_attr(docsrs, doc(cfg(...)))]` annotations that render feature
+# badges on each item.
 [package.metadata.docs.rs]
-all-features = true
+features = ["std", "hash", "dict_builder"]
 rustdoc-args = ["--cfg", "docsrs"]
 
 [dependencies]

zstd/src/encoding/bt/mod.rs

@@ -351,8 +351,10 @@ impl BtMatcher {
     /// coordinates** (so its bucket-table entries remain valid
     /// across window evictions — entries inserted by an earlier
     /// view of the frame are filtered by the staleness check
-    /// `entry.offset <= history_abs_start`). The caller is
-    /// expected to pass:
+    /// `entry.offset < history_abs_start`, i.e. inclusive lower
+    /// bound: entries at exactly `history_abs_start` survive,
+    /// see `ldm::search::FindBestMatchInputs::lowest_index_abs`).
+    /// The caller is expected to pass:
     ///
     /// * `live_history` — the contiguous *live* slice of the
     ///   per-frame `MatchTable::history` (`&history[history_start..]`).

zstd/src/encoding/ldm/mod.rs

@@ -164,9 +164,10 @@ impl LdmProducer {
     /// **absolute stream coordinates** so the bucket table stays
     /// valid across window evictions: after a slide,
     /// `history_abs_start` advances and any entry inserted by an
-    /// earlier window is filtered by the `entry.offset <=
-    /// lowest_index_abs` staleness check in
-    /// [`find_best_match`].
+    /// earlier window is filtered by the inclusive lower-bound
+    /// staleness check `entry.offset < lowest_index_abs` in
+    /// [`find_best_match`] (entries at exactly
+    /// `lowest_index_abs == history_abs_start` survive).
     ///
     /// Implements the donor pipeline from
     /// `ZSTD_ldm_generateSequences_internal`
@@ -586,8 +587,10 @@ mod tests {
     ///      entries from call 1 must remain reachable: their
     ///      absolute offsets still point at the SAME bytes
     ///      (now further left in the live slice), and the
-    ///      staleness check `entry.offset <= history_abs_start`
-    ///      keeps them in-window.
+    ///      inclusive lower-bound staleness check
+    ///      `entry.offset < history_abs_start` keeps them
+    ///      in-window (entries at exactly `history_abs_start`
+    ///      survive).
     ///
     /// If the producer had stored slice-relative indices
     /// instead, call 2 would either miss the long-range match