Load FileWatching before precompilepkgs in ensurecompiled

[KristofferC]

Fixes #1134.

Since #1131, ensurecompiled runs Base.Precompilation.precompilepkgs() under the base sysimage. On Julia 1.11 precompile_pkgs_maybe_cachefile_lock does Base.loaded_modules[FileWatching], which assumes FileWatching is already loaded. That holds for the default sysimage but not for the stdlib-free base sysimage of incremental=false builds, so every precompile task died with a KeyError that is swallowed inside the @async task, and precompilepkgs() printed Precompiling project... and exited 0 having compiled nothing. This is visible in the libtrixi CI logs: v2.4.1 precompiled 273 packages in that step, v2.4.3 spent 3 seconds and compiled none.

The --output-o step then loaded every package from source. Since Julia 1.11 jl_atexit_hook runs Base._atexit before jl_write_compiler_output, so every sysimage carries _atexit_hooks_finished = true, and a nonincremental --output-o process skips the Base.__init__ that resets it. Packages whose top-level code calls atexit, like Downloads (its precompile block calls Curl.__init__()), therefore fail with cannot register new atexit hook; already exiting. when loaded from source there. Loading from a cache file avoids running that top-level code.

Pkg.precompile() did not hit this because loading Pkg loads FileWatching. Julia 1.12+ use Base.trymkpidlock_hook with a return f() fallback when the hooks are undefined, so they are unaffected; loading FileWatching there is harmless.

The new test builds a nonincremental sysimage with Downloads and no precompile execution file, which is the only configuration where the cache files can come from ensurecompiled alone (the existing app/library tests all run a precompile script under the base sysimage, which compiles the caches through regular loading). Verified locally on Julia 1.11.9 with a fresh depot: it fails with the error from the issue without the fix and passes with it.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.19%. Comparing base (4e98b47) to head (cf008e5).

❗ There is a different number of reports uploaded between BASE (4e98b47) and HEAD (cf008e5). Click for more details.

HEAD has 6 uploads less than BASE

Flag	BASE (4e98b47)	HEAD (cf008e5)
	7	1

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1135      +/-   ##
==========================================
- Coverage   83.93%   74.19%   -9.75%     
==========================================
  Files           3        3              
  Lines         909      903       -6     
==========================================
- Hits          763      670      -93     
- Misses        146      233      +87     

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