docs(P01): complete docs foundation + firewall extension phase

P1 complete. Docs lexicon firewall (lexicon_meta_docs_test.go, 5 tests incl.
G-013 walk-coverage + G-014 shared self-test). MkDocs Material scaffold with
26-page nav (G-011). README.md + docs/index.md + 7 docs/shared/ pages. Both
firewalls green, go test ./... 22 packages green, no regression.

---ci---
project: oy
phase: 1
milestone: v0.3
status: complete
tag_base: v0.2.x
phase_role: execution
requirements:
  covered: [REQ-028]
  partial: [REQ-027]
---/ci---

docs/shared/bread-scale.md

@@ -0,0 +1,35 @@
+# Bread scale
+
+The unit of value in OpenYield is **Bread** (REQ-013). Bread is scaled in 11
+tiers, each 1,000× the previous, so a Holder can reason about a Crumb and a
+Granary in the same mental model:
+
+| Tier | Name | Multiple |
+|---|---|---|
+| 1 | **Grain** | 1 |
+| 2 | **Crumb** | 1,000 Grain |
+| 3 | **Bread** | 1,000 Crumb |
+| 4 | **Loaf** | 1,000 Bread |
+| 5 | **Batch** | 1,000 Loaf |
+| 6 | **Cake** | 1,000 Batch |
+| 7 | **Bakery** | 1,000 Cake |
+| 8 | **Granary** | 1,000 Bakery |
+| 9 | **Mill** | 1,000 Granary |
+| 10 | **Harvest** | 1,000 Mill |
+| 11 | **Earth** | 1,000 Harvest |
+
+## Why 11 tiers
+
+The 11-tier scale gives the mesh a single unit for everything from a
+1-Grain internal minimum (the Fee Covenant floor) to the Earth-tier totals
+held in the Root-Pool. There is no separate "small unit" and "large unit":
+the Bread scale is the unit. The 1-Grain minimum prevents dust games; the
+tier names keep human-readable values at every scale.
+
+## Where Bread lives
+
+Bread is held in the three [Storage Pools](storage-pools.md): the Stash
+(Holder-level), the Vault (Stand-level), and the Root-Pool (treasury). The
+Watchers attest to the state of the pools daily; the Mirror mirrors the
+canonical state to the bearers. See [Watchers & Mirror](watchers-mirror.md)
+for the attestation layer.