edit: plan every edit from the file the editor holds, not its path

FileEditor re-opened its path to plan each edit but wrote through the file
it held open, and the Python 'r+' handle re-opened the path after every
edit to read. When the path came to name another file between edits (a
rename or replacement, or a relative path after os.chdir), an edit was laid
out from the other file's metadata and written into the held one,
corrupting it, and later reads came from the other file (the review's
repro: h5py then reports "invalid dataset size, likely file corruption").

The editor now plans from a mapping of its own file (a clone of the held
descriptor, dropped before the edit writes) and canonicalises its path at
open. New FileEditor::reader() opens the held file anew for reading,
without sharing the editor's flock (a mapping of a cloned descriptor holds
the lock until unmapped): through /proc/self/fd on Linux, which follows a
renamed file; elsewhere by path, refused on Unix when the path no longer
names the held file. The Python handle reads through it and keeps no path;
a 'w' file is written at the absolute path it was opened with.

Tests: edit_tests.rs edits_go_to_the_file_held_not_the_path; test_edit.py
test_relative_path_and_chdir and test_path_replaced_between_edits.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

docs/known-issues.md

@@ -143,6 +143,12 @@ space it leaves is too small for its next, larger version.
 **No journal.** A crash while an edit patches existing structures can leave
 the file inconsistent; see the `FileEditor` documentation.
 
+**Renamed files outside Linux.** Edits always go to the file the editor
+opened. `FileEditor::reader()` (which the Python `'r+'` handle reads
+through) reopens it through `/proc/self/fd` on Linux; elsewhere it reopens
+the path, and after the path was renamed or replaced it fails (on Unix;
+Windows cannot tell and would read whatever the path names).
+
 ## Python in-place editing (`clawhdf5.File(path, 'r+')`) limits
 
 **Status:** open (added 2026-09-27). The Python bindings edit through