Move OnPair default-feature flag up to the vortex umbrella crate

Wiring `default = ["onpair"]` directly on `vortex-btrblocks` and
`vortex-file` meant any consumer that depended on those crates with
default features on (including `wasm-test`, which sets
`vortex = { default-features = false }` but cannot disable transitive
default features on a hard dep of `vortex`) ended up pulling
`vortex-onpair-sys` and its CMake / C++20 build, which fails on
wasm32-wasip1.

Move the default-on toggle to the umbrella `vortex` crate:

* `vortex-btrblocks` and `vortex-file` now declare `onpair` as a
  feature with **no `default = [...]` line** — they're a la carte.
* `vortex/Cargo.toml`: `default = ["files", "zstd", "onpair"]` plus a
  new `onpair = ["vortex-btrblocks/onpair", "vortex-file?/onpair"]`
  alias so `vortex` consumers still get OnPair out of the box but
  `default-features = false` callers (wasm-test) really do drop it.
* `only_cuda_compatible` annotates its now-conditionally-mutated
  `excluded` list with `#[cfg_attr(not(feature = "onpair"),
  allow(unused_mut))]` so no-default-features builds stop warning.

Verified:
  cargo build --target wasm32-wasip1 \
      --manifest-path wasm-test/Cargo.toml    # green, no C++ build

Signed-off-by: Claude <noreply@anthropic.com>

Author: claude

vortex-btrblocks/Cargo.toml

@@ -48,9 +48,11 @@ vortex-array = { workspace = true, features = ["_test-harness"] }
 vortex-session = { workspace = true }
 
 [features]
-default = ["onpair"]
 # This feature enabled unstable encodings for which we don't guarantee stability.
 unstable_encodings = ["dep:vortex-tensor", "vortex-zstd?/unstable_encodings"]
+# OnPair short-string compression. Pulls in a C++ build dependency (CMake +
+# C++20). Off by default so wasm / minimal-deps builds work; the umbrella
+# `vortex` crate enables it in its own defaults.
 onpair = ["dep:vortex-onpair"]
 pco = ["dep:pco", "dep:vortex-pco"]
 zstd = ["dep:vortex-zstd"]

vortex-btrblocks/src/builder.rs

@@ -173,6 +173,7 @@ impl BtrBlocksCompressorBuilder {
         // dictionary expansion at decode time, which is incompatible with
         // pure-GPU decompression paths. Strip whichever string-fragment
         // scheme is enabled by feature.
+        #[cfg_attr(not(feature = "onpair"), allow(unused_mut))]
         let mut excluded: Vec<SchemeId> = vec![
             integer::SparseScheme.id(),
             integer::IntRLEScheme.id(),

vortex-file/Cargo.toml

@@ -69,8 +69,8 @@ vortex-scan = { workspace = true }
 workspace = true
 
 [features]
-default = ["onpair"]
 object_store = ["dep:object_store", "vortex-io/object_store", "tokio"]
+# OnPair short-string compression (see vortex-btrblocks for build details).
 onpair = ["dep:vortex-onpair", "vortex-btrblocks/onpair"]
 tokio = [
     "dep:tokio",

vortex/Cargo.toml

@@ -69,10 +69,14 @@ vortex-bench = { workspace = true, features = ["unstable_encodings"] }
 vortex-tensor = { workspace = true }
 
 [features]
-default = ["files", "zstd"]
+default = ["files", "zstd", "onpair"]
 files = ["dep:vortex-file"]
 memmap2 = ["vortex-buffer/memmap2"]
 object_store = ["vortex-file/object_store", "vortex-io/object_store"]
+# OnPair short-string compression. Requires a C++ build toolchain
+# (CMake + C++20). Enabled by default but consumers can opt out via
+# `default-features = false`.
+onpair = ["vortex-btrblocks/onpair", "vortex-file?/onpair"]
 tokio = ["vortex-file/tokio"]
 zstd = ["dep:vortex-zstd", "vortex-file/zstd"]
 pretty = ["vortex-array/table-display"]