Complete phase two benchmarks, plot previews and static export workflow

backend/app/plot_routes.py

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+"""Interactive previews use the same bounded parser and geometry as exports."""
+import asyncio
+from fastapi import APIRouter
+from pydantic import BaseModel, Field
+from app.plot.parser import parse_source
+from app.plot.render import render_svg
+from app.plot.model import PlotDiagnostic, StaticRenderResult
+
+router = APIRouter(prefix='/api/plots', tags=['Function Plot'])
+_slots = asyncio.Semaphore(2)
+
+class PlotRequest(BaseModel):
+    source: str = Field(max_length=20000)
+    theme_id: str = Field(default='light', max_length=100)
+
+class PlotResponse(BaseModel):
+    result: StaticRenderResult | None = None
+    diagnostics: list[PlotDiagnostic] = Field(default_factory=list)
+    node_count: int = 0
+
+def preview(request):
+    parsed = parse_source(request.source)
+    if parsed.plot is None:
+        return PlotResponse(diagnostics=parsed.diagnostics)
+    if parsed.plot.node_count > 8000:
+        return PlotResponse(node_count=parsed.plot.node_count, diagnostics=[PlotDiagnostic(
+            severity='error', code='PLOT_BUDGET_EXCEEDED', message='图表累计表达式节点超过 8000 上限')])
+    return PlotResponse(result=render_svg(parsed.plot, request.theme_id),
+                        diagnostics=parsed.diagnostics, node_count=parsed.plot.node_count)
+
+@router.post('/function', response_model=PlotResponse)
+async def render_function(request: PlotRequest):
+    async with _slots:
+        return await asyncio.to_thread(preview, request)