fix(widget): system Cmd+V paste no longer lands at cursor+1 in vim mode

[LikeSundayLikeRain]

Summary

• @replit/codemirror-vim 6.3.0 attaches a bubble-phase paste listener on view.contentDOM that, in normal/visual mode, advances the cursor by +1 and forces insert mode (collapsing visual selection) before the native paste insert runs — so Cmd+V/Ctrl+V land at cursor+1 in normal mode and never replace a visual selection.
• New src/widget/systemPasteExtension.ts (createSystemPasteExtension()) — a ViewPlugin that attaches a capture-phase paste listener on contentDOM, reads clipboardData.getData('text/plain'), dispatches replaceSelection(text) with userEvent: 'input.paste', and calls preventDefault() + stopImmediatePropagation() to pre-empt vim's bubble handler. After insert it routes to insert mode via the public Vim.handleKey API.
• Wired into buildExtensions immediately after vimCompartment.of(...) so it's always-on (vim ON or OFF — when vim is off the capture handler produces the same result as the native default).
• Vim register-based p/P/yy/]p paste keys go through vim's internal command dispatcher (not the DOM paste event) and are untouched.

Root-cause artifacts: .planning/debug/vim-cmd-v-paste-offset.md.

Test plan

Automated (CI):

• npm run build — clean
• npm run lint — 0 errors
• npm test — 2965 passed / 8 skipped / 0 failures

Manual UAT (cannot be unit-tested without a live Obsidian instance, per CLAUDE.md):

• Visual mode replace: enter widget, v + extend, Cmd+V → selection replaced, editor in insert mode.
• Normal mode insert at cursor: cursor on a char, Cmd+V → clipboard inserted at cursor (not cursor+1).
• Vim register paste regression: yy then p → line duplicates after-cursor (unchanged).
• Cmd+V works the same with vim mode OFF (always-on extension shouldn't regress non-vim users).