Add document-container crate (#4191)

* Add document-container crate: container backends and archive codecs

* Address PR review: path/prefix split, safe size casts, OPFS stream aborts

* Address PR review round 2: mmap read check, UTF8 entry names, prefix normalization

* Make MmappedBytes::new fallible so mmap reads can't silently degrade

* Address PR review round 3: backend contract uniformity (symlinks, remove, list)

* Apply symlink-component check to FolderBackend listing paths

* Address PR review: idempotent OPFS delete logging, tar default-features, shared entry-size cap

* Fix validate_path doc: dotfiles pass, CurDir/ParentDir rejected

* Omit symlink entries from FolderBackend listings for consistency with resolve

* Preserve zip I/O errors and reject non-canonical paths in validate_path

* Extend archive apis to return the archive writer

* Rename document/document-container directory to document/container

* Add archive format sniffing and deserialize_auto

* Drop temporal hedge from checked_entry_size comment

* Tighten verbose doc comments in document-container

* Coalesce consecutive same-path OPFS appends to avoid O(n^2) file copies

* Review

* Update document-container for the deserialize/store_non_blocking renames

---------

Co-authored-by: Timon <me@timon.zip>

document/container/src/backends.rs

@@ -0,0 +1,9 @@
+//! Container backend implementations.
+
+pub mod memory;
+
+#[cfg(not(target_family = "wasm"))]
+pub mod folder;
+
+#[cfg(target_family = "wasm")]
+pub mod opfs;