Fix the dead Test-extension guard and let the extension implement a stub

[devmotion]

Two commits, kept separate so the pre-existing bug stays in the history even though the second commit deletes the line it lives on.

1. Fix the extension name in the error hint. The guard queried :InverseFunctionsTest, but the extension is registered as InverseFunctionsTestExt. Base.get_extension returns nothing for an unknown name rather than erroring, so the guard was permanently true and "Did you forget to load Test?" was appended to every test_inverse MethodError — including ones raised while Test is loaded.

2. Let the extension implement an internal stub. test_inverse becomes a regular method that forwards to _test_inverse, implemented by InverseFunctionsTestExt. That removes __init__, which is the expensive part: on Julia 1.10 (LTS) it costs 17–20 ms of load time, ~80% of it compilation, and an empty __init__ costs the same, so the cost is having one at all. Load time drops to 1.3–1.9 ms; precompilation gets cheaper too.

Compared with a Base.get_extension lookup, the stub needs no extension-name literal — the construct behind the bug in commit 1 — and stays statically resolvable, so it does not trip the --trim verifier. Arity errors go through ordinary dispatch, so no internal method leaks into "Closest candidates".

Behaviour change: a correct-arity call without Test now throws ArgumentError instead of a MethodError carrying a hint, and a wrong-arity call without Test gets a plain MethodError with no hint. The latter is inherent to removing __init__ on Julia ≤ 1.10.

Tests cover both directions, plus @test !isdefined(InverseFunctions, :__init__) so the load-time fix cannot be silently reverted. Full suite passes on 1.11; error shapes and happy path checked on 1.6, 1.10 and 1.11.

Closes #62 (same goal, different route — credit to @Beforerr for identifying the load-time cost) and closes #66 (version bumped to 0.1.18 here).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.49%. Comparing base (0434eba) to head (d51de21).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
+ Coverage   98.48%   98.49%   +0.01%     
==========================================
  Files           6        6              
  Lines         132      133       +1     
==========================================
+ Hits          130      131       +1     
  Misses          2        2              

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