Add memory dual writing and v2 readiness reporting (#43827)

## Why

Allow v2 memories to build in the background while the selected memory version continues supplying context, and let clients check whether v2 has enough consolidated data and a valid summary to use.

## What changed

- Add `memories.dual_write`, disabled by default, to run v1 and v2 extraction and consolidation concurrently with separate stores and directories.
- Add the experimental `memory/status` endpoint, returning `v2ConsolidatedThreads` and `v2Ready`. Readiness requires a valid v2 summary and a consolidation thread count meeting `minConsolidatedThreads` (default 20, supported range 1–4096).
- Persist the largest thread count from a successful consolidation across pruning and clear it on memory reset.
- Share v2 summary validation between consolidation and status reporting, and tag memory metrics with `memory_version`.

## Testing

Add tests for independent dual-write outputs, preserving existing notes without importing them into v2, and readiness behavior across consolidation ownership checks, missing summaries, configurable thresholds, thread deletion, and memory reset.

GitOrigin-RevId: 54417cc60bf419062401355c0da8c79119995f61

codex-rs/core/src/config/config_tests.rs

@@ -334,6 +334,7 @@ consolidation_model = "gpt-5.2"
         toml::from_str::<ConfigToml>(memories).expect("TOML deserialization should succeed");
     assert_eq!(
         Some(MemoriesToml {
+            dual_write: None,
             version: None,
             disable_on_external_context: Some(true),
             generate_memories: Some(false),
@@ -361,6 +362,7 @@ consolidation_model = "gpt-5.2"
     assert_eq!(
         config.memories,
         MemoriesConfig {
+            dual_write: false,
             version: codex_protocol::MemoryVersion::V1,
             disable_on_external_context: true,
             generate_memories: false,