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>

crates/kebab-app/Cargo.toml

@@ -52,6 +52,8 @@ unicode-normalization = "0.1"
 # p9-fb-31: GitignoreBuilder for .kebabignore matching in ingest_file_with_config.
 # Same version as kebab-source-fs (0.4) to avoid duplicate dep versions.
 ignore               = "0.4"
+# p9-fb-34: opaque pagination cursor encodes payload as base64.
+base64               = { workspace = true }
 
 [dev-dependencies]
 rusqlite             = { workspace = true }