[BUG] Source-view inspection of a large Unicode DOCX panics when truncating extracted text #125

Environment

Windows, Rust 1.99.1, NoteCraft 0.0.9 at 33429876e885613f6ff8f29a762b15b2be736401 (current main when tested). Verified against the real workspace libraries / front end with synthetic files and an isolated profile. No product source changes or mocks.

Reproduction

1. Create a DOCX whose word/document.xml contains one paragraph with "a".repeat(262143) + "é" as its text.
2. Attach the DOCX to a page.
3. Open Page source and select the DOCX in the file list.

The synthetic package used for verification includes [Content_Types].xml, _rels/.rels and word/document.xml. There is no personal document content. A minimal XML body is:

let text = "a".repeat(262143) + "é";
let xml = format!(
  "<w:document xmlns:w=\"http://schemas.openxmlformats.org/wordprocessingml/2006/main\"><w:body><w:p><w:r><w:t>{text}</w:t></w:r></w:p></w:body></w:document>"
);
// Store xml as word/document.xml in a DOCX ZIP.

Actual result

Selecting the file through the actual codeview::show Office branch exits with:

panicked at crates/ui-egui/src/codeview.rs:436:18:
assertion failed: self.is_char_boundary(new_len)

The byte limit 262,144 falls inside the two-byte é at 262,143..262,145.

Expected result

Show a safely shortened text preview with an explicit truncation notice. Never panic on ordinary Unicode Office text. Use a valid character boundary when applying a byte cap; retain bounded reading too.

Cause / coverage

codeview.rs, office_text, calls text.truncate(MAX_SHOWN) without checking a UTF-8 boundary. DOCX/XLSX/PPTX text share this extraction path. Add a source-view regression where a multibyte character crosses the preview cap.

This is separate from #102's link resolver and from destructive editing of a truncated page: this DOCX path is read-only but still crashes.

Duplicate check

Checked all open/closed issues and PRs, including the first audit's #102-#116 and new PRs #117-#120. No existing issue or PR covering this specific behavior was found.