#3121 conversation cache: opt-in overwrite growth for snapshot buffers

本文

## Summary

Parking a long conversation grows host K/V buffers and then overwrites the new bytes with the snapshot copy. Add an opt-in overwrite growth path to avoid initializing those bytes twice. Only `STRATA_SNAPSHOT_NOFILL=1` enables it; the default still fills new bytes. Related: #2264.

## What changed

- Store each `ConversationBuffer` segment in owned raw byte storage while preserving value semantics, address stability and ordinary resize filling.
- Add `resize_for_overwrite` and use it at the snapshot capture call site under the opt-in switch. Keep the existing cache policy and snapshot format.
- Add host tests for full overwrite, retained prefixes, segment boundaries, copy/move, allocation failure, failed transfer callbacks and discard/retry.
- Include sanitized historical v0.1.45 park timings and their limits in `bench/results/conversation-overwrite/`.

## Validation and remaining work

Standalone buffer tests on this main-based candidate: 92 checks pass with MSVC 19.44, C++17, optimized build. A separate v0.1.44.1 backport builds CUDA sm_86 and passes 92 buffer checks, 4,219 cache checks and 36 checkpoint split allocation-failure cases.

The CSV is historical evidence from one patched v0.1.45 binary and two RTX 3090 cards at a 262,144-token context. It includes a slower overwrite run under host contention; it is not a current-main or backport benchmark. The README documents exactly what is timed and omitted. No model, private path, config, prompt, binary or credential is included.

**Draft: not ready for review.** Full current-main CUDA, HIP and SYCL builds remain outstanding. HIP/SYCL toolchains are unavailable on this host. Fresh same-binary backport A/B, prefix edits, two alternating long histories, HTTP error recovery, and a NOFILL-enabled 3-hour/50-request soak are not yet complete. No production binary has been replaced. There is no claim that temperature zero makes outputs deterministic or that all failure modes have been covered.