feat(telemetry): honor DO_NOT_TRACK + print first-run notice (#166 step 5)

[BryanFRD]

Step 5 of FerrLabs-Cloud#166 — adoption of the cross-tool DO_NOT_TRACK standard and a first-run notice.

Telemetry changes

• DO_NOT_TRACK=1 now disables anonymous telemetry, alongside FERRFLOW_TELEMETRY=0 / FERRFLOW_ANONYMOUS_TELEMETRY=0. DO_NOT_TRACK wins if set to 1/true/yes/on.

• First invocation prints one line to stderr:

  Anonymous telemetry on. See https://ferrlabs.com/telemetry — opt out: FERRFLOW_TELEMETRY=0 or DO_NOT_TRACK=1

  A marker file at $XDG_STATE_HOME/ferrflow/.telemetry-notice-shown (with platform fallbacks for macOS / Windows, override via FERRFLOW_STATE_DIR) suppresses subsequent runs. We never silently re-enable telemetry.

3 new unit tests cover DO_NOT_TRACK parsing + the marker path resolver.

Drive-by clippy cleanup

The .githooks/pre-commit hook (cargo clippy --workspace --all-targets -- -D warnings) was failing on pre-existing issues that prevented any commit:

1. benches/ferrflow_benchmarks.rs — criterion::black_box deprecated, switched to std::hint::black_box and reshaped 2 call sites that violated unit_arg.
2. src/formats/{csproj,gradle,json,toml_format,txt}.rs — these put #[cfg(test)] mod tests before the impl VersionFile block (test-first reading order). The items_after_test_module lint is now allowed at the workspace level rather than reorganising 5 files.

Refs Changelog#5 entry.

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Benchmark

Details

Benchmark suite	Current: 4863998	Previous: 0b5fe57	Ratio
git_changed_since_tag/100_commits_50_since	2830392 ns/iter (± 9880)
git_changed_since_tag/1000_commits_500_since	28580851 ns/iter (± 877383)

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