Render via memegen API; Pillow fallback for custom images

[brianckeegan]

Summary

• Rewires the rendering pipeline around three backends (auto / memegen / pillow / matplotlib). By default meme() now constructs a memegen rendering URL (/images/<id>/<line>/.../<line>.<ext>?style=...&font=...), fetches the composed image, and imshows it.
• Custom local images and any feature memegen can't express (per-line fontsize, custom outlines, **text_kwargs, per-line overrides) route to a new client-side Pillow renderer. Legacy Axes.text + path-effects rendering is preserved as backend="matplotlib".
• Adds Meme.line(index, text, *, fontsize=, color=, font=, position=) for per-line overrides and Meme.with_backend(...) chainable setter.
• New meme() knobs: backend, extension, width, height, layout, background, overlays, template_style. CLI and MCP gain matching arguments.
• Template now carries lines_count / overlays_count / styles / is_memegen.
• Documents the memegen URL grammar (escape table, query parameters, fonts/styles/overlays) in a new docs/url_construction.rst, adapted from jacebrowning/memegen#993.
• Bumps to 0.5.0 with a CHANGELOG migration note. CLAUDE.md and README rewritten for the dual-backend architecture.

Why

The memegen API can compose entire memes server-side, but memeplotlib was only using it to fetch blank backgrounds and re-implementing text composition client-side with matplotlib. That left memegen features (template styles, custom fonts/colours, dimensions, layouts, custom backgrounds, overlay slots) unused. Routing the default path through the API gives users canonical memegen output for free; the Pillow fallback covers the cases the API can't reach (custom local images, explicit fontsize, custom outlines).

Reviewer notes

• Output appearance changes by default. Existing baselines are unaffected because the autouse fixture in tests/conftest.py pins legacy tests to backend="matplotlib"; new tests for the memegen and pillow paths opt in via @pytest.mark.uses_default_backend. Users who want exact v0.4 visuals can pass backend="matplotlib" or set config["backend"] = "matplotlib".
• memegen never honours custom outlines or per-line fontsize. Under auto, passing those forces the Pillow backend automatically; under backend="memegen" they are silently ignored. This is documented in the changelog and CLAUDE.md.
• Sentinel pattern in meme(): outline_color, outline_width, fontsize default to a private _UNSET sentinel so the dispatcher can tell "user passed it" from "user accepted the default". _UNSET is treated as None by every downstream consumer.
• Template.get_image() now respects config["cache_enabled"]. This was a latent bug surfaced while writing tests — under the old behaviour, disabling caching at the config level didn't actually disable the cache instance, so tests could read stale data from the user's real cache directory.
• Test mocking for memegen: rendered URLs are dynamic (the path embeds caption text). Use the new memegen_rendered_pattern("buzz") helper in tests/conftest.py for regex matching with responses.

Test plan

• ruff check .
• black --check .
• mypy --strict src/memeplotlib
• pytest --cov --mpl — 222 passed, 88% coverage (≥ 85% gate)
• sphinx-build -W docs docs/_build
• CI matrix on this PR (Python 3.10–3.13 × ubuntu/macos/windows)
• Manual smoke against the real memegen API once approved (pytest -m integration)

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