fix(image): reconcile mj-image DOM on render to avoid <img> reload/flicker

[geonsang-jo]

Fixes #431

Problem

Editing any attribute/style of an mj-image makes the rendered <img> flicker: the browser refetches and repaints the image on every edit.

Side-by-side repro on grapesjs-mjml@1.0.8 — toggle the width and watch the Baseline image reload while the Patched one stays put.

Root cause

The base MJML view re-renders by replacing the whole subtree: this.el.innerHTML = this.getTemplateFromMjml() runs on every change:attributes / change:src. That destroys and recreates the <img> node each time, so the browser drops the decoded image and fetches/repaints it.

Fix

An mj-image-only render override that reconciles the freshly compiled subtree into the existing DOM instead of replacing innerHTML: matching nodes are kept (preserving the <img> identity) and only changed attributes are written in place. It mirrors coreMjmlView.render exactly except for that one substitution, and follows the existing per-component override pattern (Column, NavBar).

• Non-src edits (alt/title/style/padding) never touch the <img>'s src, so no refetch/flicker.
• A src edit updates the same node's src in place.

Scope & known limitation

Scoped to mj-image. Adding/removing a link wrapper changes the structure (td > img ↔ td > a > img), so the index-based reconciliation replaces the <img> on that transition rather than moving it — documented in code and locked by a test so the boundary is intentional.

Tests

New tests/specs/image-dom-identity.test.ts covering node-identity preservation across attribute/src/style edits, link-wrapper add/remove, and the core no-refetch invariant (a non-src edit must not re-set src). All existing tests pass; eslint and tsc --noEmit are clean. No public API change.