aprender-test-lib fails strict clippy under its own non-default features — the trap that hid probar's dead code

[noahgift]

Found while working #2473. Separate from that issue and not fixed in #2497 or #2498.

$ cargo clippy -p aprender-test-lib --all-targets \
    --features browser,docker,llm,proptest,derive,compute-blocks -- -D warnings
rc=101

Five distinct violations, all in files neither open PR touches:

site	lint
src/tui/brick.rs:209	only a panic! in if-then statement
src/tui/compute_block.rs:171	only a panic! in if-then statement
src/docker.rs:161	from_str can be confused for std::str::FromStr::from_str
src/llm/report.rs:96	redundant closure
src/llm/score.rs:229	clamp-like pattern without using clamp

plus an unsafe block missing a safety comment in the test targets.

Why nobody noticed

Without the feature flags, the same command is clean. This is the trap already
recorded as project_theater_triage_dark_targets: "clippy --all-targets -p X is
CLEAN while X is broken behind a non-default feature." browser, docker, llm,
proptest, derive and compute-blocks are all off by default in
aprender-test-lib, and nothing in CI turns them on for a lint pass.

It is the same root cause as the probar findings themselves: code that no
configuration compiles is code nothing checks. ProbarDriver had only
MockDriver and ActionExecutor had no production impl at all, for years, behind
a feature flag CI never enabled.

Worth noting how this was measured, because it nearly went the other way: my first
run reported 0 errors and I was about to record the finding as unreproducible.
The difference was -- -D warnings. Without it clippy exits 0 and prints warnings;
the repo's actual gate uses -D warnings. A "clean" clippy run that omits the flag
the gate uses is not evidence of anything.

Suggested fix

1. Fix the five sites — all mechanical, none behavioural.
2. Add a lint pass over aprender-test-lib with its features enabled to CI, or
   the same five will come back and the next mock-only trait will hide just as
   long.

Related: #2473 (what the dark feature hid), #2495, #2496 (same family — targets
and scans that nothing exercises).