fix(pr-review-canvas): number diffs from raw hunks, never silently drop import lines - #482

Closes #472

Problem

renderDiff in cursor-team-kit/skills/pr-review-canvas/renderer.js dropped
import lines and rewrote whitespace-only del/add pairs before line numbers
were computed from the @@ hunk headers, so the rendered review was neither
the diff nor correctly numbered:

1. Import filtering shifted line numbers. Every added/removed/context line
   matching isImport — including side-effect imports like
   import './polyfill' — was deleted up front. Counters oL/nL are only
   reset by hunk headers, so every line after a dropped import within the same
   hunk was numbered too low. Issue repro:

   @@ -1,3 +1,3 @@
    import a from 'a'
   -import b from 'b'
   +import c from 'c'
    const x = 1
   

   Both changed import lines vanished and const x = 1 displayed as line 1/1
   instead of 3/3.
2. Whitespace collapsing hid real changes. isWhitespaceOnly compared
   both sides with all whitespace stripped (replace(/\s/g, '')), so
   - return "a b" / + return "ab" was rewritten into an unchanged context
   line — likewise for string-literal spacing and Python/YAML/Make
   indentation changes.

Fix

• Parse and number the full raw diff first; import lines are now tagged
  hidden instead of deleted, so line numbers always follow the original
  hunk headers.
• Hidden import lines are grouped into a visibly-marked, expandable
  "N import lines hidden" row (toggleHidden) — collapsed by default but
  never silent, and they keep their correct line numbers when expanded.
• isWhitespaceOnly now compares after trimming leading/trailing
  whitespace only, so internal whitespace differences are preserved as real
  del/add rows.
• SKILL.md and styles.css updated to document/style the new behavior.

Moved-code detection and the rest of the render pipeline are unchanged.

Verification

• node --check renderer.js — syntax clean.
• Repro harness (Node + DOM stub driving renderDiff on the issue's exact
  repro diffs): all 8 assertions pass —
  • issue repro Website design system extraction #1: const x = 1 renders as 3/3, changed import lines present
    under the "3 import lines hidden" row;
  • issue repro Audit plugin prompt capture #2: "a b" → "ab" renders as del+add, not context;
  • genuine leading/trailing-whitespace-only change still collapses to a
    context row, with following lines correctly numbered;
  • side-effect import (import './polyfill') preserved, line numbers after
    it correct;
  • multi-hunk numbering, a real 3-line move (moved-code tint), and the
    singular/plural hidden-row labels all verified.