[codex] use native process time for windows embedding snapshot

[TheGreenCedar]

Context

PR #1070 moved exact Windows process-start identity to GetProcessTimes, but its exact packaged proof still failed during the later process snapshot. The snapshot continued converting Win32_Process.CreationDate through the DMTF converter even though this host returns a System.DateTime.

What Changed

• Reused one native OpenProcess/GetProcessTimes helper for exact identity and snapshot start time.
• Kept CIM only for executable path and command line.
• Extended the existing Windows regression to verify exact tick compatibility, stable reads, and an immediate snapshot timestamp.
• Updated the Unreleased changelog entry.

How To Review

Review the Windows-only path in crates/codestory-retrieval/src/sidecar.rs. The serialized windows:<ticks> identity, PID-reuse checks, schema, dependencies, and non-Windows behavior are unchanged.

Verification

• cargo test --locked -p codestory-retrieval --lib windows_native_embedding_identity_and_snapshot_are_stable_and_compatible
• cargo test --locked -p codestory-retrieval --lib native_embedding (21 passed)
• cargo check --locked -p codestory-retrieval -p codestory-cli
• cargo clippy --locked -p codestory-retrieval --lib -- -D warnings
• cargo fmt --all -- --check
• git diff --check

No UI change; visual evidence does not apply.

Risk

Low and Windows-only. Access/query errors remain fail-closed, and only a confirmed missing PID returns None. Exact packaged Windows x64 Vulkan proof remains the merge acceptance.

Closes #1071
Refs #1052

[TheGreenCedar]

Windows live acceptance

Exact head 9ecdf8d1b18ca11e97e2e7af68f7d79df95a3620 passed a fresh isolated Windows x64 Vulkan repair with the release-built CLI (SHA-256 7731b2dc8199bdabbab1b2bf3b3f67efd1ee381d32f5afa19210ae3d040f3d40).

• The former post-spawn identity convergence failure is gone.
• Retrieval reached full; index freshness was fresh with 307/307 files and zero errors.
• gpu_proof=verified, meaningful_accelerator_work_proven=true, and the embed smoke completed in 7 ms.
• The managed runtime recorded process identity windows:639195731332411610 on PID 35032.
• AMD Radeon RX 7900 XT Vulkan0 remained accelerator-required with CPU fallback disabled.
• Focused Windows regression, all 21 native-embedding tests, retrieval/CLI check, focused clippy, format, and diff checks passed.

A second repair invocation was rejected at the existing machine-resource broker ownership gate while the proven runtime was still live; it did not reproduce the identity failure and is not widened into this snapshot fix.

Proof-scoped cleanup will run immediately after merge. PR #1052 will then rebase onto the resulting dev/codestory-next head and receive the Windows-orchestrator handoff.