Add the conformance suite, echo fixture and integration guide

Completes FLUID-WP-0007. The seven minimal-conformance requirements and
the mechanically checkable architectural invariants are asserted as
tests rather than claimed in a README, because a conformance claim
nobody re-checks is one that quietly stops being true. Only the
checkable subset of the invariants is asserted; pretending a test can
settle the rest would be worse than leaving them to review.

TestFirstVerticalSlice runs all eleven steps of Blueprint 50 with no
human steps: two revisions, explicit routing, telemetry, a cohort
dimension, detected pressure, a hypothesis, a candidate, a 90/10
experiment, fitness comparison, promotion, and a complete audit trail.
Requests per completed task fall from 5.65 to 1.00 against a 1.20
target. A companion test runs the loop twice and requires the same
verdict, since a loop whose conclusion depended on run order would be
measuring the harness rather than the interface.

The failure-containment matrix covers Blueprint 34 directly: the data
plane keeps serving with the evidence store closed, with telemetry
wedged against a sink that never returns, after a failed build, after an
experiment rollback, and with the adaptive concurrency limit saturated.

Fixes a real bug the suite exposed. Drain closed the emitter outright,
so every request after the first flush emitted into a dead emitter and
was silently lost -- the kind of fault that makes a later measurement
quietly wrong rather than loudly broken. Emitter.Flush now waits for
delivery without stopping it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014KmVxhJ35tCo7rE7UnLwWu

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 1116572@bnt-lap001
Assistant-Session: 8ba9bb93-a72a-4883-b189-2499cce5c400

examples/echo-interface/README.md

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+# echo-interface
+
+The smallest honest reproduction of the `ArchitectureBlueprint.md` §33 worked
+example, used as the conformance fixture.
+
+Two revisions of one interface over the same backend data:
+
+| Revision | Shape | Requests per completed task |
+|---|---|---|
+| **R-1** | `GET /v1/entries` only — consumers list everything and filter locally | ~3 |
+| **R-2** | adds `GET /v1/entries/latest` — the concept consumers actually wanted | 1 |
+
+R-1 is not a strawman. It is the interface a careful designer produces before
+they have seen how it is used: a clean collection resource with no special
+cases. The pressure it generates is the point — consumers repeatedly fetching
+a collection to discard all but one item is the evidence that "latest" is a
+first-class concept the contract failed to name.
+
+The fixture has no external dependencies and runs in CI. It exists so the
+revision–experiment–fitness loop is proven mechanically before a real workload
+depends on it.

examples/echo-interface/adapter.go

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+// Package echo provides the conformance fixture adapters.
+//
+// Two revisions over the same data: R-1 exposes only a collection, R-2 adds the
+// convenience resource. Both are ordinary deterministic HTTP handlers, which is
+// the point — an adapter is a normal service, and fluid-core sits in front of
+// it without asking anything of it.
+package echo
+
+import (
+	"encoding/json"
+	"net/http"
+	"sort"
+	"time"
+)
+
+// Entry is one record the fixture serves.
+type Entry struct {
+	ID        string    `json:"id"`
+	Title     string    `json:"title"`
+	CreatedAt time.Time `json:"created_at"`
+}
+
+// Data returns a deterministic set of entries.
+//
+// Fixed timestamps rather than time.Now: a conformance suite whose fixture
+// changes between runs cannot distinguish a regression from the clock.
+func Data() []Entry {
+	base := time.Date(2026, 9, 1, 12, 0, 0, 0, time.UTC)
+	return []Entry{
+		{ID: "e-1", Title: "the river reached the forge", CreatedAt: base},
+		{ID: "e-2", Title: "nine doors stayed honest", CreatedAt: base.Add(24 * time.Hour)},
+		{ID: "e-3", Title: "the reviewing side", CreatedAt: base.Add(48 * time.Hour)},
+	}
+}
+
+// NewR1 returns the collection-only adapter.
+//
+// A consumer wanting the newest entry must fetch the whole collection, sort it
+// and discard the rest. That is the interface pressure the fixture generates.
+func NewR1() http.Handler {
+	mux := http.NewServeMux()
+	mux.HandleFunc("/v1/entries", func(w http.ResponseWriter, r *http.Request) {
+		writeJSON(w, Data())
+	})
+	return mux
+}
+
+// NewR2 returns the adapter with the convenience resource added.
+func NewR2() http.Handler {
+	mux := http.NewServeMux()
+	mux.HandleFunc("/v1/entries", func(w http.ResponseWriter, r *http.Request) {
+		writeJSON(w, Data())
+	})
+	mux.HandleFunc("/v1/entries/latest", func(w http.ResponseWriter, r *http.Request) {
+		entries := Data()
+		sort.Slice(entries, func(i, j int) bool {
+			return entries[i].CreatedAt.After(entries[j].CreatedAt)
+		})
+		writeJSON(w, entries[0])
+	})
+	return mux
+}
+
+func writeJSON(w http.ResponseWriter, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	_ = json.NewEncoder(w).Encode(v)
+}

examples/echo-interface/r1.openapi.yaml

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+openapi: "3.1.0"
+info:
+  title: echo-interface
+  version: "1"
+paths:
+  /v1/entries:
+    get:
+      operationId: listEntries
+      parameters:
+        - name: limit
+          in: query
+          schema: {type: integer, minimum: 1, maximum: 100}

examples/echo-interface/r2.openapi.yaml

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+openapi: "3.1.0"
+info:
+  title: echo-interface
+  version: "2"
+paths:
+  /v1/entries:
+    get:
+      operationId: listEntries
+      parameters:
+        - name: limit
+          in: query
+          schema: {type: integer, minimum: 1, maximum: 100}
+  /v1/entries/latest:
+    get:
+      operationId: latestEntry