fix(format): bound B-tree v2 traversal against crafted files

Traversal recursed one frame per level with the depth taken from the
file (a u16), and followed child addresses without asking whether they
were shared. Two crafted inputs, both reproduced before fixing:

- A node listing itself as its own child, under a header claiming 65 535
  levels, overflowed the stack and aborted the process — SIGABRT, not an
  error a caller can handle — from under 100 bytes.
- Levels whose children all point at one shared node below reached it
  fan-out^depth times: 29.5 million records in 8 s from ~5 KB, and one
  more level would exhaust memory.

Depth is now capped at 64, as the fractal heap already was; no real tree
approaches it, since even at the minimum fan-out of two that is over
2^64 records. And traversal stops once it has produced more records
than the file has bytes to hold them — a valid tree stores each record
once in its own bytes, so this bounds shared subtrees without trusting
the header's own `total_records`. Both inputs now fail in under a
millisecond.

Every B-tree v2 user goes through this collector: dense attributes, v2
groups, shared messages and chunk indexes. To show the budget never
refuses a real file, a new interop test has HDF5 2.0 write a depth-2
chunk index with 40 000 records and reads back all 160 000 values; it
fails when the budget is deliberately made too tight.

Also corrects `BM25Index::search`, which claimed to use Block-Max WAND.
It scores exhaustively, and pruning would not help the store:
`hybrid_search` needs every score because fusion normalises over them.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

docs/known-issues.md

@@ -169,3 +169,27 @@ python3 -m venv .venv && .venv/bin/pip install h5py numpy netCDF4
 
 Set `CLAWHDF5_REQUIRE_INTEROP=1` in any automated runner so a missing
 interpreter is a failure rather than a skip.
+
+
+---
+
+## Crafted B-tree v2 structures crash or exhaust the reader
+
+**Status:** fixed on `main` (2026-09-20), after v2.6.0. **Every release up to
+and including v2.6.0 is affected.**
+
+B-tree v2 traversal (`clawhdf5-format`, `btree_v2::collect_btree_v2_records`)
+recursed one frame per level with the depth taken from the file, and followed
+child addresses without checking whether they were shared. Two consequences
+for anyone reading untrusted files:
+
+- A node that is its own child, under a header claiming 65 535 levels, overflows
+  the stack and aborts the process. The file is under 100 bytes.
+- Levels whose children all point at one node below make the traversal visit it
+  fan-out^depth times: ~30 million records from ~5 KB, and memory exhaustion one
+  level deeper.
+
+B-tree v2 backs dense attribute storage, v2 groups, shared object header
+messages and chunk indexes, so opening an object that uses any of them is
+enough. Both are now errors: depth is capped at 64, and traversal stops once it
+has produced more records than the file could physically hold.