feat(agent): new stores default to float16 embeddings

MemoryConfig::float16 now defaults to true for new stores, on
measurement: on the full LongMemEval haystack with real MiniLM
embeddings every retrieval metric matched f32 (previous commit), and at
100K the file is 48% smaller with faster checkpoints and opens.

Existing stores are unaffected: every agent store has recorded
`float16 = false` in /meta and keeps it. A test opens the v2.5.0
fixture, saves and checkpoints, and checks the embeddings are still f32
with the old rows bit-identical; another checks a new store is float16.

CLI: `create --f32` opts out; like `--f32-index` it only ever switches
the default off. `--float16` is still accepted and now a no-op.
Values beyond +-65504 are refused, so f32 remains the choice for
unnormalised vectors — the upgrade note says so.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-cli/src/main.rs

@@ -36,10 +36,13 @@ enum Commands {
         /// Accepted for compatibility; int8 is now the default
         #[arg(long, hide = true, conflicts_with = "f32_index")]
         quantized_index: bool,
-        /// Store embeddings on disk as IEEE half precision (float16): half
-        /// the bytes, about three significant digits; values must lie within
-        /// ±65504
+        /// Store embeddings as full-precision f32 instead of the default
+        /// half precision (float16: half the bytes, about three significant
+        /// digits, values within ±65504)
         #[arg(long)]
+        f32: bool,
+        /// Accepted for compatibility; float16 is now the default
+        #[arg(long, hide = true, conflicts_with = "f32")]
         float16: bool,
     },
     /// Save a memory entry (reads JSON from stdin or --json)
@@ -107,11 +110,16 @@ fn run(cli: Cli) -> Result<(), Box<dyn std::error::Error>> {
             wal,
             f32_index,
             quantized_index: _,
-            float16,
+            f32,
+            float16: _,
         } => {
             let mut config = MemoryConfig::new(cli.path.clone(), &agent_id, dim);
             config.wal_enabled = wal;
-            config.float16 = float16;
+            // As with --f32-index: only ever switch the library default off.
+            if f32 {
+                config.float16 = false;
+            }
+            let config_float16 = config.float16;
             // 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.
@@ -127,7 +135,7 @@ fn run(cli: Cli) -> Result<(), Box<dyn std::error::Error>> {
                 "embedding_dim": dim,
                 "wal_enabled": wal,
                 "quantized_index": config_quantized,
-                "float16": float16,
+                "float16": config_float16,
                 "count": mem.count(),
             });
             println!("{}", serde_json::to_string_pretty(&j)?);