docs: clarify benchmark test file setup

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

README.md

@@ -66,7 +66,9 @@ cargo test --workspace
 cargo fmt --check --all
 cargo clippy --workspace -- -D warnings
 
-# Run benchmarks (generates ~5GB test file on first run)
+# Run benchmarks (requires a pre-generated ~5GB test file)
+mkdir -p /tmp/test-logviewer
+dd if=/dev/urandom of=/tmp/test-logviewer/extreme.log bs=1M count=5000
 cargo run -p log-viewer-bench
 cargo run -p log-viewer-bench -- --quick --suites startup,render --output results.md
 
@@ -101,4 +103,6 @@ Planned but not yet built: filtering, bookmarks, sessions, search engine (stub m
 
 Custom harness comparing mmap vs pread backends across 7 suites (startup, render, jump, memory, growth, rotation, concurrent). Uses wall-clock timing with `/proc/self/` RSS and page fault metrics. Results are written as markdown tables to `benchmark-report.md`.
 
+The harness expects `/tmp/test-logviewer/extreme.log` to already exist. It does not generate the primary 5GB test file automatically; create it first with the `dd` command shown above.
+
 The benchmark binary includes ~75 unit tests for the reader backends and metrics collection.