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/btree_v1.rs

@@ -24,6 +24,21 @@ pub struct BTreeV1Node {
     pub children: Vec<u64>,
 }
 
+/// Checks that `[offset, offset + needed)` fits within `data`, guarding the
+/// addition against `usize` overflow from a crafted near-`usize::MAX` offset.
+fn ensure_len(data: &[u8], offset: usize, needed: usize) -> Result<(), FormatError> {
+    if offset
+        .checked_add(needed)
+        .is_none_or(|end| end > data.len())
+    {
+        return Err(FormatError::UnexpectedEof {
+            expected: offset.saturating_add(needed),
+            available: data.len(),
+        });
+    }
+    Ok(())
+}
+
 fn read_offset(data: &[u8], pos: usize, size: u8) -> Result<u64, FormatError> {
     let s = size as usize;
     if pos.checked_add(s).is_none_or(|end| end > data.len()) {
@@ -45,7 +60,7 @@ fn read_offset(data: &[u8], pos: usize, size: u8) -> Result<u64, FormatError> {
 
 fn is_undefined(data: &[u8], pos: usize, size: u8) -> bool {
     let s = size as usize;
-    if pos + s > data.len() {
+    if ensure_len(data, pos, s).is_err() {
         return false;
     }
     data[pos..pos + s].iter().all(|&b| b == 0xFF)
@@ -65,12 +80,7 @@ impl BTreeV1Node {
         // + left_sibling(offset_size) + right_sibling(offset_size)
         let os = offset_size as usize;
         let header_size = 8 + os * 2;
-        if offset + header_size > file_data.len() {
-            return Err(FormatError::UnexpectedEof {
-                expected: offset + header_size,
-                available: file_data.len(),
-            });
-        }
+        ensure_len(file_data, offset, header_size)?;
 
         if &file_data[offset..offset + 4] != b"TREE" {
             return Err(FormatError::InvalidBTreeSignature);
@@ -99,12 +109,7 @@ impl BTreeV1Node {
         let eu = entries_used as usize;
         let key_size = os; // For type 0, key = offset_size
         let needed = eu * (key_size + os) + key_size; // eu children + (eu+1) keys
-        if pos + needed > file_data.len() {
-            return Err(FormatError::UnexpectedEof {
-                expected: pos + needed,
-                available: file_data.len(),
-            });
-        }
+        ensure_len(file_data, pos, needed)?;
 
         let mut keys = Vec::with_capacity(eu + 1);
         let mut children = Vec::with_capacity(eu);
@@ -241,6 +246,16 @@ mod tests {
         assert_eq!(node.right_sibling, None);
     }
 
+    #[test]
+    fn parse_near_usize_max_offset_rejected_without_overflow() {
+        let data = build_btree_node(0, 0, &[0, 5, 10], &[0x100, 0x200], None, None, 8);
+        let result = BTreeV1Node::parse(&data, usize::MAX - 4, 8, 8);
+        assert!(
+            matches!(result, Err(FormatError::UnexpectedEof { .. })),
+            "expected a clean UnexpectedEof, got {result:?}"
+        );
+    }
+
     #[test]
     fn parse_with_siblings_none() {
         let data = build_btree_node(0, 0, &[0, 8], &[0x300], None, None, 8);