Count the sealed-to-unsealed fallback, and offer to refuse it (ios#16)

🔴 **Off by default, and that is the honest default rather than the lazy
one.** An unsealed send is still end-to-end encrypted; what leaks is
that this account sent something to that one. Refusing turns a metadata
leak into an undelivered message, which for most people is the worse
outcome — so the default degrades and says so, and the refusal is there
for the people who would rather a message did not go than go
attributable.

The counters answer a question nobody could ask before. `Sent.sealed`
says whether one send was sealed; the record says how often this happens
to you, which is what somebody actually needs to decide whether the
switch is worth throwing.

⚠️ Refusing costs nothing: `sealed` is false exactly when neither a
capability nor a key was found, so there is no token to hand back.

⚠️ Both the policy and the counters are wired in ONE helper called from
both sign-in paths. Two call sites building a `Session` and only one
applying the policy is how a setting silently stops working on one of
them.

Taken by hand rather than cherry-picked — the original also rewrote a
ChatScreens that has since changed underneath it. The settings switch
for it is UI work still to come.

engine/src/main/kotlin/io/flamenet/e2e/SealingRecord.kt

@@ -0,0 +1,65 @@
+package io.flamenet.e2e
+
+/**
+ * How often sending falls back from sealed to unsealed (§14.5, ios#16).
+ *
+ * 🔴 **The fallback is the thing worth counting.** Sealing hides WHO SENT a
+ * message from the relay. When the peer's delivery key or capability is missing
+ * the send still goes — end-to-end encrypted as always — but unsealed, so the
+ * relay learns the sender. The outcome was reported per send and nothing
+ * accumulated, so nobody could answer "how often does this happen to me".
+ *
+ * ⚠️ **Aggregate only, deliberately.** Counting per contact would be more
+ * useful and would also build a record of who this account talks to and how
+ * often — the precise thing sealing exists to keep off a server, kept instead
+ * on the device. Totals answer the question without producing a social graph as
+ * a side effect.
+ */
+data class SealingRecord(
+    /** Sends where the relay could not see who sent it. */
+    val sealed: Int = 0,
+    /** Sends where it could. */
+    val unsealed: Int = 0,
+    /** When the last unsealed send happened, unix seconds. 0 = never. */
+    val lastUnsealedAt: Long = 0,
+) {
+    val total: Int get() = sealed + unsealed
+
+    /**
+     * What share of sends the relay could attribute, 0..1.
+     *
+     * ⚠️ Zero sends is not "nothing leaked", it is "nothing sent". Returning
+     * 0.0 would render as a reassuring 0% on a fresh install.
+     */
+    val unsealedShare: Double? get() =
+        if (total == 0) null else unsealed.toDouble() / total.toDouble()
+
+    /**
+     * The sentence shown to the user.
+     *
+     * 🔴 **Says what is exposed, not that the message was readable.** An
+     * unsealed send is still end-to-end encrypted: the relay learns who sent
+     * it, not what it said. Copy that blurred the two would either frighten
+     * people about nothing or, worse, teach them that "sealed" means
+     * "encrypted" and leave them relaxed when it is off.
+     */
+    val summary: String get() {
+        val share = unsealedShare ?: return "Nothing sent from this device yet."
+        if (unsealed == 0) {
+            return "All $sealed messages sent from this device were sealed — " +
+                "the server was never told who sent them."
+        }
+        val percent = Math.round(share * 100).toInt()
+        return "$unsealed of $total messages sent from this device ($percent%) went " +
+            "unsealed. Those were still end-to-end encrypted — the server could see " +
+            "who sent them, not what they said."
+    }
+
+    /** Fold one send's outcome in. */
+    fun recording(wasSealed: Boolean, at: Long): SealingRecord =
+        if (wasSealed) copy(sealed = sealed + 1)
+        // ⚠️ A sealed send must not clear the stamp: "when did this last
+        // happen" is the question, and one good send afterwards does not
+        // unhappen it.
+        else copy(unsealed = unsealed + 1, lastUnsealedAt = at)
+}

engine/src/test/kotlin/io/flamenet/e2e/SealingRecordTest.kt

@@ -0,0 +1,69 @@
+package io.flamenet.e2e
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFalse
+import kotlin.test.assertNull
+import kotlin.test.assertTrue
+
+/** Counting the sealed-to-unsealed fallback (§14.5, ios#16). */
+class SealingRecordTest {
+
+    @Test
+    fun `a sealed send is counted`() {
+        val r = SealingRecord().recording(wasSealed = true, at = 5)
+        assertEquals(1, r.sealed)
+        assertEquals(0, r.unsealed)
+    }
+
+    @Test
+    fun `an unsealed send is counted and stamped`() {
+        val r = SealingRecord().recording(wasSealed = false, at = 1_700_000_000)
+        assertEquals(1, r.unsealed)
+        assertEquals(1_700_000_000, r.lastUnsealedAt)
+    }
+
+    /** ⚠️ One good send afterwards does not unhappen the last bad one. */
+    @Test
+    fun `a sealed send does not clear the last unsealed stamp`() {
+        val r = SealingRecord()
+            .recording(wasSealed = false, at = 100)
+            .recording(wasSealed = true, at = 200)
+        assertEquals(100, r.lastUnsealedAt)
+    }
+
+    /** 🔴 Zero sends is no data, not zero leakage. */
+    @Test
+    fun `no sends has no share rather than zero`() {
+        assertNull(SealingRecord().unsealedShare)
+        assertTrue(SealingRecord().summary.contains("Nothing sent"))
+    }
+
+    @Test
+    fun `the share is what went unsealed`() {
+        var r = SealingRecord()
+        repeat(3) { r = r.recording(wasSealed = true, at = 1) }
+        r = r.recording(wasSealed = false, at = 1)
+        assertEquals(0.25, r.unsealedShare!!, 0.0001)
+    }
+
+    /**
+     * 🔴 **Must not say the message was readable.** An unsealed send is still
+     * end-to-end encrypted; the relay learns the sender, not the content.
+     */
+    @Test
+    fun `the summary distinguishes sender from content`() {
+        val summary = SealingRecord().recording(wasSealed = false, at = 1).summary
+        assertTrue(summary.contains("end-to-end encrypted"), summary)
+        assertTrue(summary.contains("who sent them"), summary)
+        assertTrue(summary.contains("not what they said"), summary)
+    }
+
+    @Test
+    fun `a clean record says so without overclaiming`() {
+        var r = SealingRecord()
+        repeat(5) { r = r.recording(wasSealed = true, at = 1) }
+        assertTrue(r.summary.contains("were sealed"), r.summary)
+        assertFalse(r.summary.contains("%"), r.summary)
+    }
+}