[tmm]

The Rust port had no @lexical/file, no @lexical/devtools-core tree
view and none of @lexical/text's root helpers, so file/export,
file/import and devtools/tree were reported as not implemented.

This adds them, each behind its own Cargo feature:

• file: serializedDocumentFromEditorState,
  editorStateFromSerializedDocument, and importFile/exportFile, with
  the file picking and saving left to the app through callbacks, as Swift
  does.
• devtools: generateContent (the tree view). Its link branch is built
  with link.
• text: $rootTextContent, $isRootTextContentEmpty,
  $canShowPlaceholder (with their curried forms) and
  $findTextIntersectionFromCharacters.
• Core: NodeRef::is_selected, NodeRef::is_before, Editor::VERSION,
  get_namespace, is_editable/set_editable, and a namespace field on
  CreateEditorArgs that defaults to a createUID()-style id. The meta
  suite checks that Editor::VERSION is lexical-rust@<UPSTREAM_VERSION>.

No vector reached the root helpers, so packages/lexical-conformance gains
four script actions that call the real @lexical/text functions and a
JS-recorded scripts/text vector (13 cases: empty documents, whitespace
String#trim does and doesn't remove, text outside the BMP, blocks, indent,
line breaks, tabs, headings, quotes, nested lists and decorators). Swift
replays it too; Swift's LexicalText already matched JS.

Not ported, each listed in PARITY.md: the devtools exportDOM form (needs
@lexical/html), its mark and table branches, slots and the
compositionKey line, and the DOM halves of importFile/exportFile.

$ cargo run -q --locked --release -p lexical-conformance
| `devtools/tree` | – | – | – | – | – | not implemented |
| `file/export` | – | – | – | – | – | not implemented |
| `file/import` | – | – | – | – | – | not implemented |
| **Total** | 569 | 569 | 0 | 0 | 0 | |

$ cargo run -q --locked --release -p lexical-conformance
| `devtools/tree` | 38 | 38 | 0 | 0 | 0 | pass |
| `file/export` | 8 | 8 | 0 | 0 | 0 | pass |
| `file/import` | 3 | 3 | 0 | 0 | 0 | pass |
| `scripts/text` | 13 | 13 | 0 | 0 | 0 | pass |
| **Total** | 631 | 631 | 0 | 0 | 0 | |

Mutation checks (each reverted): devtools ^ padding off by one fails 10
devtools/tree cases; is_selected always false fails 25; an undefaulted
empty source fails 2 file/export cases; Rust's trim() instead of JS's
whitespace fails 1 scripts/text case; >= for > in
find_text_intersection_from_characters fails 9; a wrong Editor::VERSION
fails meta.

cargo fmt --all --check, cargo clippy --workspace --all-targets --all-features --locked -- -D warnings, cargo test --workspace --all-features --locked, cargo doc with -D warnings, cargo +1.88 check -p lexical-editor --all-features, --no-default-features alone and
with file, devtools, text and devtools,link, and the Rune app's
clippy all pass. UPDATE_VECTORS=1 pnpm exec vitest run src/scripts.test.ts: 19 passed, and no other vector changed (md5).
Swift: ScriptTests, FileTests and TreeViewTests pass.