#3335 [Other] Serialize conversation-cache log polling

Description

## Problem and result

Concurrent `/metrics` readers could read the same conversation-cache log range and replay an event. This change serializes access to the shared log cursor, so direct and HTTP metrics readers count each event once.

## Related discussion

Continues #3041 after the 0.2.0-beta tracker reset. The original passed the new format check, but GitHub refused to reopen the rebased branch; [the reset bot](https://github.com/Niko1221/Strata/pull/3041#issuecomment-6282591930) explicitly suggested a new linked pull request.

Closes #3039.

## Approach

Guard the existing log-poll operation with the cache object's lock and add direct-reader and local loopback `/metrics` concurrency regressions. Cache parsing and native inference behavior are unchanged.

## Validation

Rebased on upstream main `e88ee39d1d6be6e3364183af3b39b62a7ed6f211` (0.2.0-beta); tested candidate `04796fa0427beaa6b8628d6e2ab914c7059ca8c0` on Windows, Python 3.14.3. Each test command starts a fresh local process.

**Passed:**
- `py -3.12 -m ruff check --output-format json serve/server.py serve/test_monitor.py` (Ruff 0.2.1): 10 existing diagnostics, same as unmodified main; zero new diagnostics.

- `python -m unittest -v serve.test_monitor serve.test_prometheus` — 18 tests passed.
- `python -m py_compile serve/server.py serve/test_monitor.py`.
- `git diff --check e88ee39d1d6be6e3364183af3b39b62a7ed6f211..HEAD`.

**Failed:**
- With the exact base production file restored, the direct concurrency test replayed the park event twice. The initial two-test run also had one environment error before the local `regex` dependency was installed. The subsequent fresh loopback HTTP `/metrics` red test failed as expected with `(parks, restores) = (2, 0)` instead of `(1, 0)`.

**Not run:**
- No GPU, native backend, model download, or hardware inference test.

## Limits

The proof covers concurrent Python readers, the repository mock engine, and a local loopback HTTP server. It does not exercise a real model process or hardware runtime.