fix(blossom-bridge): pick rightmost sha256 segment in URL path

CDNs like share.yabu.me serve blobs under `<cache-prefix-sha>/<blob-sha>.<ext>`
where both path segments are 64-char hex. The bridge previously locked onto the
first match (the cache prefix), dropping the blob segment from `xs` and asking
the local cache for a non-existent blob. Walking the path right-to-left makes
the rightmost sha — the one that carries the file extension — win, while the
prefix segments stay in `xs` so the cache can fetch upstream on miss.

Behaviour is unchanged when the path has a single sha; tests cover the new
two-hash layout in both the Coil-model path and the OkHttp interceptor path.

amethyst/src/main/java/com/vitorpamplona/amethyst/service/okhttp/LocalBlossomCacheRedirectInterceptor.kt

@@ -72,8 +72,13 @@ class LocalBlossomCacheRedirectInterceptor(
     }
 
     private fun findSha256AndExtensionInPath(url: HttpUrl): Triple<Int, String, String>? {
-        url.pathSegments.forEachIndexed { index, segment ->
-            val match = SHA256_SEGMENT_REGEX.find(segment) ?: return@forEachIndexed
+        // Walk segments right-to-left so CDNs that put a cache prefix (itself a
+        // 64-char hex segment) ahead of the blob hash — e.g.
+        // `https://share.yabu.me/<prefix>/<sha>.webp` — still resolve to the
+        // blob and leave the prefix in `xs`.
+        for (index in url.pathSegments.indices.reversed()) {
+            val segment = url.pathSegments[index]
+            val match = SHA256_SEGMENT_REGEX.find(segment) ?: continue
             val sha = match.value.lowercase()
             val ext = guessExtensionFrom(segment, sha) ?: "bin"
             return Triple(index, sha, ext)

amethyst/src/test/java/com/vitorpamplona/amethyst/service/okhttp/LocalBlossomCacheRedirectInterceptorTest.kt

@@ -132,6 +132,21 @@ class LocalBlossomCacheRedirectInterceptorTest {
         response.close()
     }
 
+    @Test
+    fun bridgeOnPicksRightmostShaWhenPathHasTwoHashes() {
+        // share.yabu.me layout: <cache-prefix-sha>/<blob-sha>.<ext>
+        val prefix = "84b0c46ab699ac35eb2ca286470b85e081db2087cdef63932236c397417782f5"
+        val blob = "28fa4d999af6ae3e4e11bfc2727130ef1b3a13cc0f981e5a93c3996cb2f524e5"
+        val interceptor = LocalBlossomCacheRedirectInterceptor { true }
+        val captured = mutableListOf<String>()
+        val response = interceptor.intercept(fakeChain("https://share.yabu.me/$prefix/$blob.webp", captured))
+        assertEquals(
+            "http://127.0.0.1:24242/$blob.webp?xs=https%3A%2F%2Fshare.yabu.me%2F$prefix",
+            captured.single(),
+        )
+        response.close()
+    }
+
     private fun fakeChain(
         url: String,
         captured: MutableList<String>,