docs: document the pdf.js viewer breakage on the Qt 5.15 / Windows 7 build

The vendored pdf.js v3.11.174 legacy bundle targets ~Chrome 92, while Qt
5.15.2's QtWebEngine is Chromium 83. Logical assignment (||=, ??=, &&=) and
private/static-private methods therefore fail to parse, so build/pdf.js never
defines pdfjsLib and the viewer stays empty.

Add src/data/extra/web/pdf.js/AGENTS.md with the symptom, the cause, the cost of
each fix route, the syntax-gap check to run first, and a warning that the file
name in the JS console log lines is unreliable. Correct the misleading 'Why v3
and not v4+' note in README.md and index the new doc from the root AGENTS.md.

AGENTS.md

@@ -365,6 +365,7 @@ put the full detail in the module doc.
 | Widgets | [src/widgets/AGENTS.md](src/widgets/AGENTS.md) | Widget conventions, ViewArea2 framework, styling, construction pattern |
 | GUI Services | [src/gui/AGENTS.md](src/gui/AGENTS.md) | Theme, ViewWindowFactory, GUI utilities |
 | Utilities | [src/utils/AGENTS.md](src/utils/AGENTS.md) | PathUtils, HtmlUtils, FileUtils2 reference |
+| pdf.js assets | [src/data/extra/web/pdf.js/AGENTS.md](src/data/extra/web/pdf.js/AGENTS.md) | Touching the vendored PDF viewer, or triaging "the PDF viewer is broken on the Qt 5.15 / Windows 7 build" |
 | Testing | [tests/AGENTS.md](tests/AGENTS.md) | Writing/running tests in either suite, test mode, fixtures, coverage |
 | CI & Packaging | [.github/AGENTS.md](.github/AGENTS.md) | Workflows, `src/Packaging.cmake`, Windows 7 / Qt 5.15 variant, bundled OpenSSL |
 | vxcore (submodule) | [libs/vxcore/AGENTS.md](libs/vxcore/AGENTS.md) | C library: notebook/config/search backend |

src/data/extra/web/pdf.js/AGENTS.md

@@ -0,0 +1,109 @@
+# pdf.js (vendored) — VNote PDF viewer assets
+
+Vendoring rules, VNote-owned files and the upgrade procedure live in
+[README.md](README.md) next to this file (it is the file a non-agent contributor reads first).
+This document holds the knowledge that is NOT obvious from the vendored tree.
+
+---
+
+## KNOWN BROKEN: the PDF viewer does not work on the Qt 5.15 / Windows 7 build
+
+**Status: known, accepted, NOT fixed.** Do not file it as a regression, and do not
+"fix" it with a one-off patch to a vendored file (see [Do not hand-patch](#do-not-hand-patch-the-vendored-bundles) below).
+
+### Symptom
+
+Opening any PDF in the `win64-windows7` (Qt 5.15.2) package logs three errors and shows an
+empty viewer:
+
+```
+Critical:(...:4003) Uncaught SyntaxError: Unexpected token '='
+Critical:(...:2729) Uncaught SyntaxError: Unexpected token '='
+Critical:(...:29)   Uncaught ReferenceError: pdfjsLib is not defined
+```
+
+Qt 6 builds (Windows/Linux/macOS) are unaffected.
+
+### Cause
+
+| | |
+|---|---|
+| Vendored viewer | pdf.js **v3.11.174**, the **legacy** dist (verified byte-identical to the upstream `pdfjs-3.11.174-legacy-dist.zip` — this is not a mis-vendoring) |
+| pdf.js v3 legacy browser target | ~Chrome 92 |
+| Qt 5.15.2 QtWebEngine | **Chromium 83** |
+
+The bundle therefore contains syntax Chromium 83 cannot **parse**:
+
+| Feature | Shipped in | Occurrences (`build/pdf.js` + `web/viewer.js` + `pdf.worker.js`) |
+|---|---|---|
+| logical assignment `\|\|=`, `??=`, `&&=` | Chrome 85 | 22 + 23 + 41 |
+| private methods / `static #x` | Chrome 84 | 299 + 307 + 95 |
+
+The two `SyntaxError`s in the log are exactly `build/pdf.js:4003`
+(`(intentState.renderTasks ||= new Set())`) and `web/viewer.js:2729`
+(`this._contentDispositionFilename ??= …`). Because `build/pdf.js` fails to parse as a whole,
+the global `pdfjsLib` is never created, which is the third error (raised from
+`pdfviewer.js:29`, `pdfjsLib.GlobalWorkerOptions.workerSrc = …`).
+
+The scripts are injected as plain `<script src>` tags by
+`HtmlTemplateService::fillResources`, from the list in
+[`src/core/pdfviewerconfig.cpp`](../../../core/pdfviewerconfig.cpp)
+(`PdfViewerConfig::defaultViewerResource`). Nothing in VNote's own JS is at fault, and
+`pdfviewercore.js` / `pdfviewer.js` themselves are ES5-compatible.
+
+### Do not trust the file name in those log lines
+
+`web/js/markdownviewer.js` is 80 lines long, so `markdownviewer.js:4003` cannot be real.
+QtWebEngine's default `javaScriptConsoleMessage` builds a `QMessageLogContext` from a
+**temporary** UTF-8 buffer of `sourceID`, so the `const char *file` VNote's `Logger` prints can
+be a dangling/recycled pointer naming some other script. **The line numbers are accurate; the
+file name is not.** (`WebPage::javaScriptConsoleMessage`, `src/widgets/webpage.cpp:34`, only
+intercepts `InfoMessageLevel`; warnings and errors fall through to the Qt default.)
+
+### Why it is not fixed
+
+Every route costs more than the platform is worth today:
+
+| Option | Cost |
+|---|---|
+| Transpile the 4 bundles with `@babel/preset-env` `targets: chrome 83` and vendor the output | A ~4 MB vendored diff that no longer matches any upstream artifact, plus a Node/Babel step in the vendoring procedure |
+| Downgrade to pdf.js v2.16.105 (last v2; its legacy dist is ES5) | Re-derive `web/pdf-viewer-template.html` from v2's `viewer.html`, re-verify the `pdfviewercore.js` outline bridge, new CSS/images/locales — and it downgrades the viewer on *every* platform |
+| Ship a second, Chromium-83-safe asset tree selected per build | ~10 MB of duplicate assets, CMake/packaging branching, two code paths to test |
+
+Users on the Qt 5.15 / Windows 7 package should open PDFs in an external viewer.
+
+### If you do decide to fix it
+
+The check to run first is a syntax-only one; do not assume `||=` is the whole gap:
+
+```powershell
+rg -c "(\|\||\?\?|&&)=" src/data/extra/web/pdf.js/build/pdf.js src/data/extra/web/pdf.js/web/viewer.js
+rg -c "#[a-zA-Z_]+\(|static #"  src/data/extra/web/pdf.js/build/pdf.js src/data/extra/web/pdf.js/web/viewer.js
+```
+
+`build/pdf.worker.js` and `build/pdf.sandbox.js` must be converted together with the two
+above — the worker parses in its own realm and fails independently of the main frame.
+
+---
+
+## Do not hand-patch the vendored bundles
+
+`build/*.js`, `web/viewer.js`, `web/viewer.css`, `web/locale/`, `web/cmaps/`,
+`web/standard_fonts/` and `web/images/` are upstream artifacts and are replaced wholesale on
+upgrade. A local edit inside them is silently lost at the next bump and has no test guarding
+it. The VNote-owned files are only the five listed in [README.md](README.md)
+(`pdfviewer.js`, `pdfviewercore.js`, `pdfviewer.css`, `web/pdf-viewer-template.html`, and
+`README.md` itself). Any behavioural change belongs in those, or in a documented,
+reproducible post-processing step applied to the whole vendored tree.
+
+---
+
+## Related
+
+| Where | What |
+|---|---|
+| [README.md](README.md) | Upstream source, version, VNote-owned files, what is deliberately not vendored |
+| [`src/core/pdfviewerconfig.cpp`](../../../core/pdfviewerconfig.cpp) | The script/style load order injected into the template |
+| [`src/core/services/htmltemplateservice.cpp`](../../../core/services/htmltemplateservice.cpp) | `VX_STYLES_PLACEHOLDER` / `VX_SCRIPTS_PLACEHOLDER` substitution |
+| [`src/widgets/editors/pdfvieweradapter.cpp`](../../../widgets/editors/pdfvieweradapter.cpp) | C++ side of the QWebChannel outline bridge |
+| [`.github/AGENTS.md`](../../../../.github/AGENTS.md) | The Windows 7 / Qt 5.15 packaging variant this limitation is scoped to |

src/data/extra/web/pdf.js/README.md

@@ -19,5 +19,9 @@ across an upgrade:
 Not vendored from the ZIP: `*.map` source maps, `web/debugger.{js,css}`,
 `web/compressed.tracemonkey-pldi-09.pdf`, and the top-level `LICENSE`.
 
-Why v3 and not v4+: Qt 5.15's QtWebEngine is Chromium 83, and pdf.js's legacy
-build requires Chrome 125+ from v4 on. v3.11.174 is the last v3 release.
+Why v3 and not v4+: pdf.js's legacy build requires Chrome 125+ from v4 on, which
+no Qt version VNote ships can meet. v3.11.174 is the last v3 release.
+
+Note that v3's legacy build targets ~Chrome 92 and therefore does NOT run on
+Qt 5.15's QtWebEngine (Chromium 83) either: the PDF viewer is known broken on the
+win64-windows7 package. See AGENTS.md in this folder before touching anything here.