refactor(lexicon): shared SyntheticBannedStrings() helper (REQ-029, G-014)

Add lexicon.SyntheticBannedStrings() — single source for the synthetic
self-test table consumed by BOTH meta-tests. Refactor
TestLexiconMetaSelfTestTable (lexicon_meta) and TestLexiconMetaDocsSelfTestTable
(lexicon_meta_docs) to consume the helper; remove the byte-identical
duplicated 10-string table from both. Closes the G-014 drift risk.

test(hub): cross-package const-equality test (REQ-030, A-304, G-015)

Add x/hub/types/cross_const_test.go: test-only import of x/bond/types
(G-003 test-exempt). TestLendingCouponCapMatchesBondCap + TestLendingCoupon-
FloorMatchesBondFloor assert cross-equality; TestConstsAreMissionLocked800And0
(G-015) asserts absolute 800/0 values — catches paired drift. Closes A-304.

Verification: go test ./... green; old synthetic table gone (grep 0);
go.mod unchanged; G-003 production firewall intact.

---ci---
project: oy
phase: 1
milestone: v0.4
status: execute
tag_base: v0.3.x
milestone_type: nfr
reqs: [REQ-029, REQ-030]
---/ci---

x/hub/types/cross_const_test.go

@@ -0,0 +1,70 @@
+package types
+
+// cross_const_test.go (REQ-030, REVIEW.md P2 / A-304, GRILL G-015) is a
+// cross-package const-equality test that catches silent drift between the
+// x/hub LOCAL consts (LendingCouponCapBps / LendingCouponFloorBps) and the
+// x/bond mission-locked consts (CouponCapBps / CouponFloorBps, D-028).
+//
+// Before REQ-030, the two const pairs were cross-documented only by a comment
+// (x/hub/types/types.go:46-55) — no automated check existed. A future
+// mission-locked change to x/bond.CouponCapBps without a matching x/hub change
+// would silently drift. This test fails closed on either kind of drift:
+//
+//   - single-sided drift: hub stays 800, bond changes to 900 → the equality
+//     test fails.
+//   - paired drift: BOTH change to the same wrong value (e.g., both 900) → the
+//     equality test passes BUT the absolute-value test (G-015) fails, because
+//     the mission-locked value is 800, not 900.
+//
+// G-003 (no production cross-module struct imports): this is a TEST-ONLY
+// import of x/bond/types in a _test.go file. G-003's test-import exemption
+// (documented in v0.2 GRILL G-003 and already exercised by
+// x/bearers/types/types_test.go:7 importing x/processing/types) permits
+// cross-package test imports. NO production .go file in x/hub/types/ imports
+// x/bond/types (the P1-99-01 verification greps non-test .go files to confirm).
+
+import (
+	"testing"
+
+	bondtypes "github.com/oy/openyield/x/bond/types"
+)
+
+// TestLendingCouponCapMatchesBondCap asserts the x/hub LOCAL
+// LendingCouponCapBps equals the x/bond mission-locked CouponCapBps (A-304).
+// Fails on single-sided drift (one changes, the other does not).
+func TestLendingCouponCapMatchesBondCap(t *testing.T) {
+	if LendingCouponCapBps != bondtypes.CouponCapBps {
+		t.Errorf("A-304 drift: x/hub LendingCouponCapBps = %d, x/bond CouponCapBps = %d (must match)", LendingCouponCapBps, bondtypes.CouponCapBps)
+	}
+}
+
+// TestLendingCouponFloorMatchesBondFloor asserts the x/hub LOCAL
+// LendingCouponFloorBps equals the x/bond mission-locked CouponFloorBps
+// (A-304). Fails on single-sided drift.
+func TestLendingCouponFloorMatchesBondFloor(t *testing.T) {
+	if LendingCouponFloorBps != bondtypes.CouponFloorBps {
+		t.Errorf("A-304 drift: x/hub LendingCouponFloorBps = %d, x/bond CouponFloorBps = %d (must match)", LendingCouponFloorBps, bondtypes.CouponFloorBps)
+	}
+}
+
+// TestConstsAreMissionLocked800And0 (GRILL G-015) asserts the ABSOLUTE
+// mission-locked values: both caps are 800 (8pct, D-028) and both floors are 0
+// (0pct, D-028). This catches PAIRED drift — if both consts change to the same
+// wrong value (e.g., both 900), the equality tests above pass but this test
+// fails, because the mission-locked value is 800, not 900. The 8pct cap /
+// 0pct floor is the anti-greed covenant (vision §17, §18); defending the
+// absolute value is the highest-priority regression guard in v0.4.
+func TestConstsAreMissionLocked800And0(t *testing.T) {
+	if LendingCouponCapBps != 800 {
+		t.Errorf("G-015: x/hub LendingCouponCapBps = %d, want 800 (mission-locked 8pct, D-028)", LendingCouponCapBps)
+	}
+	if bondtypes.CouponCapBps != 800 {
+		t.Errorf("G-015: x/bond CouponCapBps = %d, want 800 (mission-locked 8pct, D-028)", bondtypes.CouponCapBps)
+	}
+	if LendingCouponFloorBps != 0 {
+		t.Errorf("G-015: x/hub LendingCouponFloorBps = %d, want 0 (mission-locked 0pct, D-028)", LendingCouponFloorBps)
+	}
+	if bondtypes.CouponFloorBps != 0 {
+		t.Errorf("G-015: x/bond CouponFloorBps = %d, want 0 (mission-locked 0pct, D-028)", bondtypes.CouponFloorBps)
+	}
+}