Register RRFS 2dfld product after NOAA file split

[jacob1299]

Context

Around 2026-04-16, NOAA began publishing a new 2dfld (2D surface / post-processed fields) product to the RRFS S3 bucket alongside prslev. Both files now appear for every hourly cycle:

rrfs_a/rrfs.20260417/12/rrfs.t12z.2dfld.3km.f001.conus.grib2   (HEAD 200)
rrfs_a/rrfs.20260417/12/rrfs.t12z.prslev.3km.f001.conus.grib2  (HEAD 200)

Same URL shape, same resolution, all non-NA domains (CONUS/AK/HI/PR). Not yet reflected on the AWS registry page, but live in the bucket.

Problem

Herbie(date, model="rrfs", product="2dfld", fxx=0) fails at core.py:_validate():

AssertionError: `product` must be one of {'natlev', 'testbed', 'ififip', 'prslev'}

because rrfs.template() doesn't register the key.

Fix

• Register "2dfld" in rrfs.PRODUCTS (src/herbie/models/rrfs.py)
• Mention it in the HELP docstring and the RRFS gallery notebook
• Add a template-validation test (tests/test_rrfs.py)

No URL logic changes — self.product is already interpolated into the existing filename pattern.

Verification

• End-to-end: Herbie(datetime(2026, 4, 17, 12), model="rrfs", product="2dfld", fxx=1) now resolves GRIB2 + IDX on AWS
• pytest tests/test_rrfs.py — 11/11 pass