sparsemap 4.0.0

# Changelog

All notable changes to the Rust `sparsemap` crate are documented here.
The format follows [Keep a Changelog](https://keepachangelog.com), and
the crate follows [SemVer](https://semver.org).

## [4.0.0] - 2026-06-04

### Fixed

- **Data loss for indices `>= 2^32`** (parity with C `sparsemap` 4.0.0):
  the wire format's chunk-start offsets are now 8 bytes (`u64`) so the
  serialized form addresses the full 64-bit universe the API has always
  advertised.  Previously, `to_bytes()` returned `EncodeError::IndexTooLarge`
  for any bit at or above `2^32`; now it succeeds for any valid index.

### Changed (breaking)

- The on-disk **wire format version** is now `2`.  Buffers written by
  the C `sparsemap` 4.0.0+ deserialize transparently; pre-4.0 buffers
  (v1, 4-byte starts) are rejected with `DecodeError::UnsupportedVersion(1)`
  and need to be re-serialized through C 4.0.0+ first.
- `SparseMap::to_bytes` is now infallible: signature changes from
  `Result<Vec<u8>, EncodeError>` to `Vec<u8>`.  Callers must drop the
  `?`/`.unwrap()`/`.expect()` they had on the result.
- `EncodeError` is removed (the `2^32` cap it expressed no longer
  exists).

### Verified

- Read-direction wire compatibility against checked-in fixtures emitted
  by C 4.0.0's `sm_serialize`.
- Write-direction wire compatibility (`ci/wire_compat.sh`): C 4.0.0
  reads byte-for-byte the buffers Rust produces, recovering identical
  bit sets across six representative shapes.
- `clippy::pedantic` and `rustfmt` clean; `no_std` build green.

## [3.0.1] - 2026-06-03

### Changed

- The crate is now **100% Rust with zero dependencies and no build
  script**.  3.0.0 carried a `cc` build-dependency and a `build.rs`
  that compiled the C library for an in-tree wire-compatibility test;
  a default `cargo add sparsemap` pulled and compiled `cc` (a no-op for
  consumers, since no C source ships in the crate).  Both are removed.
- Cross-language wire compatibility is still verified: the read
  direction (decoding C output) is now an in-crate, pure-Rust test
  against checked-in fixtures produced by the C `sm_serialize`
  (`ci/gen_fixtures.c`); the write direction (the C library reading
  Rust's `to_bytes`) runs in CI via `ci/wire_compat.sh`, which is
  excluded from the published crate.

No API or behavior changes.

## [3.0.0] - 2026-06-03

First release of the Rust port.  The version is aligned with the C
library's 3.0.0 so that "sparsemap 3.0.0" means the same thing in either
language.

### Added

- `SparseMap`: a safe, `no_std`-compatible sparse bitset over `u64`.
- Core operations: `insert`, `remove`, `contains`, `clear`,
  `cardinality`, `is_empty`, `min`, `max`, `rank`, `select`, `span`,
  `insert_range`, `remove_range`, `shifted`.
- Set algebra: `union`, `intersection`, `difference`,
  `symmetric_difference`, `intersects`, `is_subset`, `is_superset`,
  plus the `|`, `&`, `-`, `^` operators and their assigning forms.
- Iteration: `iter` (ascending), `to_vec`, and `IntoIterator` for
  `&SparseMap`.
- Standard traits: `Clone`, `Debug`, `Default`, `PartialEq`, `Eq`,
  `Hash`, `FromIterator<u64>`, `Extend<u64>`.
- `to_bytes` / `from_bytes`: serialization wire-compatible with the C
  library, with `EncodeError` / `DecodeError`.
- `forbid(unsafe_code)`; `no_std` with an `alloc` dependency and an
  optional `std` feature for `std::error::Error` impls.

### Verified

- Property tests against a `BTreeSet` oracle (stateful model, set
  operations, serialization round-trip, parse robustness).
- Cross-implementation wire-format tests: maps serialized by the C
  library deserialize here, and vice versa.
- Clean under `clippy::pedantic` and `rustfmt`.