chore: Tier 2 quick wins — version skew, docs, cleanup, overflow-safe bounds

- Fix version skew: clawhdf5-py (pyproject.toml 1.93.0 -> 2.1.0) and
  packages/clawhdf5-node (package.json 2.0.0 -> 2.1.0) were both behind
  the actual crate version.
- Correct stale ROADMAP.md claims: the TypeScript bridge already has a
  complete napi-rs package (not "no package.json"); CI/CD is now wired
  up via .gitea/workflows/ci.yml.
- Fix CLAUDE.md: clawhdf5-gpu uses wgpu with hand-written WGSL compute
  shaders, not CubeCL.
- chunked_read.rs: drop 12 unnecessary chunk_dimensions[..rank].to_vec()
  allocations — all three callees already accept &[u32].
- btree_v1.rs: add an overflow-safe ensure_len(data, offset, needed)
  helper (checked_add) and use it at the two plain-arithmetic bounds
  guards, closing a usize-overflow edge case reachable from a crafted
  near-usize::MAX B-tree offset. Add a regression test.
- Clarify that the integrity hashes in clawhdf5-agent/provenance.rs
  (FNV-1a) and clawhdf5-format/provenance.rs (SHA-256) are unkeyed and
  only detect accidental corruption, not tampering — doc-only change.
- README.md: document that the mpi-io feature's read/write paths are
  root-read+broadcast / gather-to-rank-0, not true collective I/O.

crates/clawhdf5-format/src/provenance.rs

@@ -2,6 +2,9 @@
 //! data-integrity verification.
 //!
 //! Enable with the `provenance` Cargo feature (on by default).
+//!
+//! The hash is unkeyed, so this detects accidental corruption only — it is
+//! not a tamper-evidence or authenticity guarantee. See [`verify_dataset`].
 
 #[cfg(not(feature = "std"))]
 use alloc::{format, string::String, vec::Vec};
@@ -115,6 +118,11 @@ pub enum VerifyResult {
 ///
 /// `file_data` is the entire HDF5 file bytes; `header` is the parsed object
 /// header for the dataset of interest.
+///
+/// This only detects *accidental* corruption. The hash is unkeyed and stored
+/// alongside the data it protects, so anyone able to modify the dataset can
+/// also recompute and overwrite `_provenance_sha256` — a `VerifyResult::Ok`
+/// is not a tamper-evidence or authenticity guarantee.
 pub fn verify_dataset(
     file_data: &[u8],
     header: &ObjectHeader,