Let a host page say it saves exported PNGs itself

[CSSFrancis]

In an iframe, the export menu's Save PNG… can't download, so it posts the image to the parent and shows an in-figure preview captioned "Right-click the image → Save image as…". A desktop app that embeds figures (SpyDE, on Electron) has its own Save dialog and a webview with no such right-click menu, so that preview left users with no way to save.

Change

• The parent can post {type: 'anyplotlib_host', savesPng: true} to the figure's frame. The page template's PNG_HARVEST_LISTENER records it in globalThis.__aplHostSavesPng, accepting it from window.parent only.
• When set, a framed save only posts {type: 'anyplotlib_export_png_result', requestId: null, dataUrl, width, height, filename}. There's no preview, and no Save as… (choose folder) row, because the host's dialog already chooses the folder (_hostSavesPng, _canPickFile).
• Hosts that don't announce anything behave as before. The navigated embed gets the listener too, since it installs the same block.

Docs

• docs/exporting.rst documents the message a framed save posts and the host announcement.
• FIGURE_ESM.md describes the host-save path. The insertion shifted 26 anchors; I shifted them by the diff hunks and checked every function anchor against its declaration line (105 checked, no mismatches).
• The line count in AGENTS.md is updated to 12,438.

Tests

• New tests/test_embed/test_host_saves_png.py, run in a parent page with the figure in an iframe:
  • without an announcement: the image is posted and the preview is shown;
  • with one: the image is posted, there's no preview and no "choose folder" row (this test fails on main);
  • a message from the frame itself, rather than its parent, is ignored.
• tests/test_embed, tests/test_plotxy and tests/test_documentation pass locally.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.20%. Comparing base (b38ca67) to head (0da5c94).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #78   +/-   ##
=======================================
  Coverage   91.20%   91.20%           
=======================================
  Files          41       41           
  Lines        4880     4880           
=======================================
  Hits         4451     4451           
  Misses        429      429           

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[Copilot]

🟢 Approval recommended

The host-save path is documented and tested, with existing behavior preserved.

Pull request overview

Adds an opt-in host-managed PNG export path for framed figures, preserving existing behavior when no host announcement is made.

Changes:

• Handles parent-only host announcements.
• Suppresses preview and folder-picker UI for host-managed saves.
• Adds tests and documentation updates.

File summaries

File	Description
upcoming_changes/78.new_feature.rst	Adds the changelog entry.
docs/exporting.rst	Documents the host export protocol.
anyplotlib/tests/test_embed/test_host_saves_png.py	Tests announced and unannounced host behavior.
anyplotlib/FIGURE_ESM.md	Updates renderer documentation and anchors.
anyplotlib/figure_esm.js	Implements host-aware PNG export behavior.
anyplotlib/_repr_utils.py	Adds the host announcement listener.
AGENTS.md	Updates renderer line-count metadata.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 0
• Review effort level: Lite

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