feat(agent): new stores use the int8 vector index by default

`MemoryConfig::quantized_index` now defaults to `true`. It holds a
quarter of the index memory and, with the exact re-score, is faster at
equal recall on every configuration measured: 1.63x the queries per
second on x86-64 (AVX2) and 1.18x on a Raspberry Pi 5 (NEON SDOT), with
builds 1.8x and 2.3x faster. The one argument for keeping it off — that
int8 search was slower on ARM — did not survive being measured.

Existing stores do not change. A store written with v2.6.0 or later
keeps its persisted setting. One written before the setting existed has
no stored value, and it loads as `false` rather than as the new default,
so reopening it never changes how its index is held. That case is
guarded by a real store written with the v2.5.0 CLI, committed as
`tests/fixtures/store_v2_5_0.h5` (6.8 KB): the test asserts it reopens
with an f32 index and still searches, and it fails if the load default
is changed to `true`.

The CLI needed more than a new default. `create --quantized-index`
assigned its value straight into the config, so under the new default
every CLI-created store would have been forced back to f32 unless the
caller knew to ask for int8. It is replaced by `--f32-index`, which only
ever switches the default off; `--quantized-index` is still accepted,
hidden, as a no-op, and the two conflict.

The whole agent suite passes under the new default, including the
brute-force recall oracle, now running on int8 plus re-score without
being asked to.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

crates/clawhdf5-cli/src/main.rs

@@ -28,9 +28,13 @@ enum Commands {
         /// Enable write-ahead log
         #[arg(long)]
         wal: bool,
-        /// Store the vector index's copy of the embeddings as int8, roughly
-        /// halving a loaded store's memory at about 13% fewer queries/second
+        /// Hold the vector index's copy of the embeddings as f32 instead of
+        /// the default int8 (which uses a quarter of the memory and is faster
+        /// at equal recall)
         #[arg(long)]
+        f32_index: bool,
+        /// Accepted for compatibility; int8 is now the default
+        #[arg(long, hide = true, conflicts_with = "f32_index")]
         quantized_index: bool,
     },
     /// Save a memory entry (reads JSON from stdin or --json)
@@ -96,11 +100,18 @@ fn run(cli: Cli) -> Result<(), Box<dyn std::error::Error>> {
             agent_id,
             dim,
             wal,
-            quantized_index,
+            f32_index,
+            quantized_index: _,
         } => {
             let mut config = MemoryConfig::new(cli.path.clone(), &agent_id, dim);
             config.wal_enabled = wal;
-            config.quantized_index = quantized_index;
+            // Only ever switch *off* the library default: assigning the flag
+            // outright would force every CLI-created store back to f32 unless
+            // the caller knew to ask for int8.
+            if f32_index {
+                config.quantized_index = false;
+            }
+            let config_quantized = config.quantized_index;
             let mem = HDF5Memory::create(config)?;
             let j = serde_json::json!({
                 "status": "created",
@@ -108,7 +119,7 @@ fn run(cli: Cli) -> Result<(), Box<dyn std::error::Error>> {
                 "agent_id": agent_id,
                 "embedding_dim": dim,
                 "wal_enabled": wal,
-                "quantized_index": quantized_index,
+                "quantized_index": config_quantized,
                 "count": mem.count(),
             });
             println!("{}", serde_json::to_string_pretty(&j)?);