docs(milestone): complete OpenYield v0.3 (Bearers & Documentation)

Milestone v0.3 complete. Feature type, tags v0.2.x. Two work-streams
shipped under one feature milestone:

(A) Bearers skeleton + tests (D-020 pattern, 7 x/* packages, zero ext deps):
- x/bridge (NEW): BridgeStatus enum (4), BridgeRoute by-ID-string refs
- x/exit (NEW): ExitStatus enum (5), ExitRoute, DEXSwap (opaque venue)
- x/bearers (EXT): OYSATLink surveillance-resistant LOCKED, OYQRCode idempotent
- x/partner (EXT): AnchorCredential (custody-provider-id empty in skeleton)
- x/hub (NEW): HubService enum (3), LendingCouponCapBps=800 LOCAL const (A-304)
- x/services (NEW): ServiceKind enum (4), window-id by-ID-string ref (A-307)
- x/bond (EXT): GrowthBond, ClampGrowth G-012 underflow guard, secondary market
All packages >=93.3% coverage. Both lexicon firewalls green. G-003 intact.

(B) Documentation deliverable (REQ-027 complete, 26-page MkDocs Material site):
- README.md + mkdocs.yml + docs/index.md
- docs/shared/ (7 pages): Six Principles, Bread Scale, Storage Pools, Watchers/Mirror, Lexicon, Vision
- docs/nomads/ (8 pages): Reach, Stash, Bearers, Maps-Pay, Pacts, Standing, Window
- docs/freeholders/ (8 pages): Signals, Standing, Stands-Guilds, Councils-Voice, Bonds, Partner Spectrum, Anchor Preview
- docs/reference/ (2 pages): Architecture, Components
- REQ-028: lexicon firewall extended to docs/ + README.md (lexicon_meta_docs_test.go, 5 tests incl G-013 walk-coverage + G-014 shared self-test)

Phases: P0 -> v0.2.0, P1 -> v0.2.1, P2 -> v0.2.2, P3 -> v0.2.3, P4 -> v0.2.4, P5 -> v0.2.5, P6 -> v0.2.6 (milestone release).

Requirements covered: REQ-010, REQ-022, REQ-023, REQ-024, REQ-025, REQ-026 (skeleton), REQ-027, REQ-028 (complete).
IDEATE-01..08 ratified and delivered.

---ci---
project: oy
phase: 6
milestone: v0.3
status: complete
tag_base: v0.2.x
phase_role: final
milestone_complete: true
requirements:
  covered: [REQ-010, REQ-022, REQ-023, REQ-024, REQ-025, REQ-026, REQ-027, REQ-028]
  partial: []
---/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.