feat: add negotiated notebook media exports

[peter-gy]

Notebook exports can now select nested values through one shared selector grammar and render figures, charts, and displayable values in the media format a consumer requests.

The new media exporter negotiates PDF, SVG, PNG, and other display methods while preserving deterministic rendering metadata such as image display dimensions. Browser image mounts use those dimensions to keep high-density images at their intended figure size. The selector and representation rules are shared by live resolution and prepared exports, with updated examples and reference documentation.

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[cubic-dev-ai]

3 issues found across 46 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. When an issue isn't valid or won't be fixed in this PR, reply in its thread with the reason and then resolve the thread. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="docs/reference/representations.md">

<violation number="1" location="docs/reference/representations.md:26">
P3: This row limits `media` to figures and charts, but `represent()` also exports values implementing IPython MIME methods or marimo’s `_mime_()`. Include displayable values so users can discover this supported input class.</violation>
</file>

<file name="packages/python/src/marimo_export/_execution/plan.py">

<violation number="1" location="packages/python/src/marimo_export/_execution/plan.py:739">
P2: This import shadows a notebook definition named `_marimo_export_select`, so selecting that value passes the helper function to `select_value` instead. Use an alias that cannot match `selector.root` and update the generated call accordingly.</violation>
</file>

<file name="docs/reference/python-api.md">

<violation number="1" location="docs/reference/python-api.md:27">
P2: This row tells users that the `[charts]` extra covers "chart media", but the new `media` exporter distributes over `("altair", "matplotlib", "vl-convert-python")` (see `exporters/_definitions.py`) while the `charts` extra only installs `altair` and `vl-convert-python` (and `[all]` also omits `matplotlib`). A user whose `ExportSpec` uses the `media` exporter on a matplotlib figure will follow this doc to install `[charts]`, then hit the ImportError from `_optional.py` because matplotlib is never installed by any extra. Either add `matplotlib` to the `charts`/`all` extras in `packages/python/pyproject.toml`, or narrow the wording on this row.</violation>
</file>

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