test: incremental coverage improvement iteration

[unhappychoice]

Summary

• Bump line/branch coverage across cache.rs, config.rs, display.rs, image_display.rs, main.rs, steam/{client,error,models,native}.rs via 50+ incremental commits (≈+4,250 / −22 LoC, almost entirely test code).
• Sole production-code change: demo_stats in src/main.rs is now pub(crate) so the new mod tests can call it (the "visibility tweak" the PR description allows).
• New #[cfg(test)] src/test_support.rs module exposing lock_env() — a single crate-wide mutex used by every env-mutating test sub-module, with poisoning recovered via into_inner so a panic in one test cannot cascade-fail unrelated tests.

Highlights

• steam/client.rs — adds an in-process HTTP server (spawn_n_shot_server / spawn_sequential_server) to drive request_with_retry through 200 / 400 / 403 / 429 / 500 / connection-refused / timeout / truncated-body / retry-then-success paths.
• steam/native.rs — covers NativeSteamClient::try_new None / load-failure / CreateInterface symbol-missing branches via a system-lib symlink, plus query_cell_size_ioctl short-circuits.
• config.rs / image_display.rs / display.rs / cache.rs — exercise XDG_CACHE_HOME / HOME env precedence, create_default_config parent-missing & write-failure branches, AchievementCache serde round-trip incl. NaN/Inf, and image cache fs round-trips.
• main.rs — adds demo_stats invariants (descending playtime, achievements ≤ possible, etc.) and Cli clap parsing tests.

Test plan

• cargo build --tests — clean
• cargo clippy --all-targets --all-features — clean
• cargo fmt -- --check — clean
• cargo test (default parallel scheduler) → 266 passed, 0 failed across 30 consecutive runs
• CI on this branch: all 7 checks green (Check / Format / Clippy / Test / Build ubuntu+macos+windows)

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: f15e5333-82fb-4a8f-91e6-b7970af76e56

📥 Commits

Reviewing files that changed from the base of the PR and between 5a04225 and 2c8890a.

📒 Files selected for processing (10)

• src/cache.rs
• src/config.rs
• src/display.rs
• src/image_display.rs
• src/main.rs
• src/steam/client.rs
• src/steam/error.rs
• src/steam/models.rs
• src/steam/native.rs
• src/test_support.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch coverage-up-20260503-062728

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