feat: add the Loaf registry

[CameronBrooks11]

Loaves were absent from the site. The daily index generation covered Slice_* and Grain_* repos but not Loaf_* — so the two real backplanes never appeared, while the Grain registry, which has no repos at all, did.

Changes

• scripts/generate_loaf_index.py — builds _data/loaf-index.json from loaf.yaml in each Loaf_* repo, mirroring the Slice and Grain generators.
• _pages/loaves.md — Loaf Registry, grouped by role.
• pages.yml — runs the loaf generator in the daily build.
• grains.md — nav_order 2 → 3, so Projects reads Slice, Loaf, Grain.

Grouped by role, not category

role is the axis that matters for a Loaf: a backplane provides Slice attachment; a controller provides the controller interface and chains onto a backplane, with no Slice slots of its own. The table surfaces slot count and bus type.

Verified against the live org

Run against the Loaf branches (the manifests aren't on main yet):

Loaf_x004    role=backplane   slots=4  bus=i2c
Loaf_ESPT    role=controller  slots=0  bus=i2c

Sorting puts backplanes before controllers, as intended.

Against main it currently returns 0 loaves — correct, since loaf.yaml lands with the Loaf remediation PRs. The index regenerates on the daily cron once those merge.

Depends on

bread-infra #9 (merged) — the Loaf schema — and the loaf.yaml manifests in the two Loaf repos' open PRs.