feat(cli): accept ffmpeg-style rational fps (NTSC, PAL, slow-mo)

Replaces the rigid `--fps 24|30|60` whitelist with a numeric range and
adds support for ffmpeg-style fractional framerates so NTSC stays exact
end-to-end.

- `--fps 30` keeps working (integer fps)
- `--fps 30000/1001` now means exact NTSC 29.97 (not the lossy decimal)
- `--fps 24000/1001`, `--fps 60000/1001`, `--fps 25/50/120/240` all work
- Decimals like `--fps 29.97` are rejected with a friendly error pointing
  the user at the rational form, since `29.97` and `30000/1001` round
  to different framerates inside ffmpeg

Carries an `Fps = { num: number; den: number }` rational end-to-end:
RenderConfig, EncoderOptions, StreamingEncoderOptions, CaptureOptions,
DockerRenderOptions, Studio API request body, regression-harness
meta.json. The `-r` and `-framerate` ffmpeg args emit the rational form
verbatim (`30000/1001`) so no decimal round-trip happens at the encoder
boundary. Frame-interval math uses `1000 * den / num` ms (33.366… for
NTSC, 33.333… for integer 30).

Helpers live in @hyperframes/core:
- `parseFps(input: string | number): FpsParseResult` — discriminated
  parser used by both the CLI and the Studio API route
- `fpsToFfmpegArg(fps: Fps): string` — emits "30" or "30000/1001"
- `fpsToNumber(fps: Fps): number` — for arithmetic (telemetry, frame
  count, frame-index → time)

Studio API wire format accepts polymorphic `fps: number | string`:
- number → integer fps (`30`)
- string → rational (`"30000/1001"`)
Decimals are rejected; matches the same rule as the CLI.

Existing meta.json fixtures with integer `"fps": 30` continue to load
unchanged — the regression-harness validator now normalizes both number
and string inputs through `parseFps`.

packages/engine/src/services/chunkEncoder.ts

@@ -17,6 +17,7 @@ import {
 } from "../utils/gpuEncoder.js";
 import { type HdrTransfer, getHdrEncoderColorParams } from "../utils/hdr.js";
 import { formatFfmpegError, runFfmpeg } from "../utils/runFfmpeg.js";
+import { fpsToFfmpegArg } from "@hyperframes/core";
 import type { EncoderOptions, EncodeResult, MuxResult } from "./chunkEncoder.types.js";
 
 export type { EncoderOptions, EncodeResult, MuxResult } from "./chunkEncoder.types.js";
@@ -105,7 +106,7 @@ export function buildEncoderArgs(
     options = { ...options, hdr: undefined };
   }
 
-  const args: string[] = [...inputArgs, "-r", String(fps)];
+  const args: string[] = [...inputArgs, "-r", fpsToFfmpegArg(fps)];
   const shouldUseGpu = useGpu && gpuEncoder !== null;
 
   if (codec === "h264" || codec === "h265") {
@@ -309,7 +310,7 @@ export async function encodeFramesFromDir(
   }
 
   const inputPath = join(framesDir, framePattern);
-  const inputArgs = ["-framerate", String(options.fps), "-i", inputPath];
+  const inputArgs = ["-framerate", fpsToFfmpegArg(options.fps), "-i", inputPath];
   const args = buildEncoderArgs(options, inputArgs, outputPath, gpuEncoder);
 
   return new Promise((resolve) => {
@@ -432,7 +433,7 @@ export async function encodeFramesChunkedConcat(
     const inputPath = join(framesDir, framePattern);
     const inputArgs = [
       "-framerate",
-      String(options.fps),
+      fpsToFfmpegArg(options.fps),
       "-start_number",
       String(startNumber),
       "-i",