feat(app): cursor encode/decode for paginated search (fb-34)

Opaque base64(JSON{offset, corpus_revision}). Mismatch or
malformed input returns ErrorV1 with code = stale_cursor.
base64 promoted to workspace dep.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -80,6 +80,7 @@ rmcp         = { version = "1.6", default-features = false, features = ["server"
 # a tokio runtime to host its mock server (the runtime adapter crate stays
 # sync via reqwest::blocking — wiremock is dev-only there).
 wiremock     = "0.6"
+base64       = "0.22"
 
 # Disk-footprint trim for dev / test builds. Codegen, opt-level, and
 # behavior are unchanged — only DWARF debug info is reduced (line