feat(producer): support entryFile for rendering individual compositions (#42)

## Changes

- Add optional `entryFile` parameter to render API endpoints (`/v1/render` and `/v1/render-stream`)
- Enable rendering individual sub-compositions by extracting them from index.html context when the entry file is a `<template>` wrapper
- Change base64 audio/video linting from detecting "fabricated" media to prohibiting all inline base64 media
- Add manifest path resolution for bundled producer deployments

## API Changes

- `RenderConfig` — new optional `entryFile` field for specifying HTML file to render
- `server.ts` — parses `entryFile` from request body, validates file exists in project directory
- `executeRenderJob` — uses `entryFile` instead of hardcoded `"index.html"`

## Template Extraction

- `extractStandaloneEntryFromIndex` — extracts sub-composition hosts from index.html and creates standalone render context
- Handles `<template>` entry files by finding matching `data-composition-src` in index.html and isolating that host
- Resets `data-start` to 0 for standalone rendering

## Linting Updates

- Change rule #3.7 from detecting "fabricated" base64 media to prohibiting all inline base64 audio/video
- Lower detection threshold from 100+ to 20+ base64 characters
- All base64 media now triggers error severity with clearer messaging about file size bloat

## Usage

```json
POST /v1/render-stream
{ "projectDir": "/path/to/project", "entryFile": "compositions/intro.html" }
```

Omit `entryFile` for default behavior (renders `index.html`).

packages/producer/src/server.ts

@@ -69,6 +69,7 @@ interface RenderInput {
   workers?: number;
   useGpu: boolean;
   debug: boolean;
+  entryFile?: string;
 }
 
 interface PreparedRenderInput {
@@ -91,7 +92,12 @@ function parseRenderOptions(body: Record<string, unknown>): Omit<RenderInput, "p
         ? body.output
         : null;
 
-  return { outputPath, fps, quality, workers, useGpu, debug };
+  const entryFile =
+    typeof body.entryFile === "string" && body.entryFile.trim().length > 0
+      ? body.entryFile.trim()
+      : undefined;
+
+  return { outputPath, fps, quality, workers, useGpu, debug, entryFile };
 }
 
 async function prepareRenderBody(
@@ -104,8 +110,9 @@ async function prepareRenderBody(
     if (!existsSync(absProjectDir) || !statSync(absProjectDir).isDirectory()) {
       return { error: `Project directory not found: ${absProjectDir}` };
     }
-    if (!existsSync(resolve(absProjectDir, "index.html"))) {
-      return { error: `No index.html in project directory: ${absProjectDir}` };
+    const entry = options.entryFile || "index.html";
+    if (!existsSync(resolve(absProjectDir, entry))) {
+      return { error: `Entry file "${entry}" not found in project directory: ${absProjectDir}` };
     }
     return { prepared: { input: { projectDir: absProjectDir, ...options } } };
   }
@@ -317,6 +324,7 @@ export function createRenderHandlers(options: HandlerOptions = {}): RenderHandle
       workers: input.workers,
       useGpu: input.useGpu,
       debug: input.debug,
+      entryFile: input.entryFile,
       logger: log,
     });
 
@@ -428,6 +436,7 @@ export function createRenderHandlers(options: HandlerOptions = {}): RenderHandle
         workers: input.workers,
         useGpu: input.useGpu,
         debug: input.debug,
+        entryFile: input.entryFile,
         logger: log,
       });
       const abortController = new AbortController();