refactor: remove the ECharts renderer

[jvdd]

Removes the deprecated ECharts renderer and the compat shims and dead code that it and older specs left behind. Closes #126.

Change

• ECharts is removed (first commit). flexviz/adapters/echarts_adapter.py, flexviz/adapters/js/echarts/ (3 files), the ECharts bundle in runtime.py, the ECharts entries in registry.py, the ECharts-only tests, and the ECharts parts of Architecture.md, docs/guides/customizing.md, docs/guides/cross-filtering.md and the docstrings.
• The renderer switch stays (option B in Remove the deprecated ECharts renderer, and decide what stays of the renderer switch #126). renderer= on Figure.show and Dashboard.show, and ?renderer= on /view and /h/{n}, keep "plotly" as the only value. adapters/registry.py stays as the place for a second renderer. The name must now match "plotly" exactly: normalize_renderer_name (lowercase and strip) is removed.
• Tests:
  • The "echarts" value is removed from every renderer parametrization, and 11 ECharts skips are gone. Two tests check that renderer=echarts gets 400 at /view and /h/{n}.
  • The single-value renderer parametrizations and the unused renderer arguments of the test helpers are removed. _wait_for_init keeps its argument for now, because fix(cube): brush binned sources on their own bin grid #134 adds calls to it in test_browser_cube.py.
  • New tests in test_adapters.py cover the capability check in validate_dashboard_renderer, check that PLOTLY_TRACE_TYPES holds every registered trace type, and check that renderer="Plotly" raises.
• Compat shims and dead code are removed:
  • the old-spec x/y branch in BarPlot.from_trace_spec, with its two tests;
  • the params.bar_mode fallback in plotly_adapter.py and plotly/traces.js;
  • the ::bg/::fg id shim in stripLayerSuffix, the legacy hover-mode coerce in toolbar.js, and the _fvClearAllCrosshairs and window.fvClearAllCrosshairs aliases;
  • _registered_sources, FlexTrace.is_grouped_parent, CorrHeatmap.default_color_scale / default_color_range, set_cache_backend and set_cube_cache_backend (no callers);
  • the call to _fvResetRendererCache in runtime/overlay.js, which only ECharts defined.
• Docs fixes:
  • Architecture.md gives spec version 0.7 (it said 0.5).
  • Architecture.md lists the 9 renderer hooks that the shared runtime and toolbar.js call, plus _fvAllFigUids. Before, it listed 4 hooks, and 3 of them do not exist after this PR. The runtime.py docstring points to this list.
  • Architecture.md no longer lists dispatchCrosshairs. The comments in pie.py and line.py point at the right format and file.
  • CHANGELOG.md records the ECharts removal, the exact renderer name and the removed cache setters. The stale ECharts sentence in the fix(server): answer only same-origin pages and loopback host names #131 entry is removed.

Impact

• 34 files changed, 526 insertions, 3,959 deletions. Most insertions are test signatures that ruff wraps again.
• No spec change. A Plotly user who passes renderer="plotly" or no renderer sees no change.
• renderer="echarts" and renderer="Plotly" now raise ValueError. A share URL or flexviz history entry with renderer=echarts returns HTTP 400 at /view and /h/{n}.
• flexviz.cache.set_cache_backend and set_cube_cache_backend are removed. They shipped in 0.1.0b1 to 0.1.0b5 but were not documented.
• feat(adapters): add a light and dark house theme #137 (stacked on this PR) needs a rebase and one fix commit: 4 of its new browser test calls pass "plotly" to helpers that no longer take it.

Checks

• pytest tests flexviz_polars/tests (what make test runs): 2,363 passed, 2 skipped.
• pytest -m browser -p no:randomly -n 6 tests (what make test-browser runs): 306 passed.
• ruff format --check, ruff check flexviz tests, mkdocs build --strict, git diff --check: pass.

[codspeed]

Merging this PR will not alter performance

✅ 31 untouched benchmarks

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_{Comparing refactor/remove-echarts (07fffcc) with main (835c799)}