Phase C: declare gateway↔BoJ-gnosis seam in Trustfile [SEAMS]

Per the Phase A contract (`docs/integration/http-capability-gateway-boj-contract.md`)
and ADR 0004, the http-capability-gateway sits in front of BoJ's unified
Zig API gnosis handler as tier-2 of the rate-limit + capability-enforcement
architecture. Declaring the seam closes Phase C / C3 of `standards#98`.

The E2E seam test lives in the gateway repo
(`web-ecosystem/http-capability-gateway:test/e2e_boj_integration_test.exs`)
because the gateway is the side that owns the policy pipeline and the proxy.
The seam declaration here makes the boundary discoverable from the BoJ
side and lets the Seams.Policy.idr proof reference it.

Refs hyperpolymath/standards#91
Closes hyperpolymath/standards#98

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: hyperpolymath

.machine_readable/contractiles/trust/Trustfile.a2ml

@@ -455,6 +455,15 @@ seams:
     failure_mode: "fail-closed (supervisor restart)"
     tier: "elixir-disciplined"
     notes: ""
+  - id: "gateway-boj-gnosis"
+    description: "http-capability-gateway ↔ BoJ unified Zig API gnosis handler"
+    from: "web-ecosystem/http-capability-gateway"
+    to: "src/zig-api/ (gnosis handler)"
+    contract: "docs/integration/http-capability-gateway-boj-contract.md"
+    test_ref: "web-ecosystem/http-capability-gateway:test/e2e_boj_integration_test.exs"
+    failure_mode: "fail-closed (circuit breaker)"
+    tier: "elixir-disciplined"
+    notes: "Phase C (hyperpolymath/standards#98). Tier-2 HTTP governance seam declared per ADR 0004; the gateway resolves trust at the transport (mTLS, Phase B) and forwards an authoritative X-Trust-Level to the gnosis handler. E2E seam test lives in the gateway repo because that is the side that owns the policy pipeline and the proxy."
 policy:
   seam_check: "every PR that touches either side of a declared seam MUST exercise the P2P test for that seam before merge"
 security: