v0.0.8: convert the extension to a read-only PDF viewer

[suzdalenko-dev]

Summary

• convert the VS Code custom editor provider to the read-only provider contract
• remove every PDF change, save, save-as, export, backup, undo, and redo path
• replace the editor webview with navigation, thumbnails, zoom, search, and password support only
• reduce the MuPDF facade to non-mutating load, authentication, render, bounds, thumbnail, and search operations
• remove Fabric.js and the obsolete save-mode setting
• bump the extension and documentation to 0.0.8

Read-only guarantees

• the extension host has no workspace.fs.writeFile path
• the webview protocol allow-lists only ready and show-message
• unknown/editing messages are rejected
• MuPDF journaling and serialization are not enabled or exposed
• the VSIX contains no Fabric asset or editing canvas

Verification

• npm run check — lint, typecheck, and 12/12 tests passed
• npm run package — generated pdf-viewer-editor-0.0.8.vsix
• packaged images/icon.png verified
• packaged contents checked for the absence of Fabric/editing assets

This PR is intentionally left unmerged for review.