Release the autoload claim when Autoload#load has nothing to load

[aminmansuri]

Autoload#load records the claim (ctx) before it checks featureAlreadyLoaded. When the feature is already loaded or still loading there is nothing to load, but the claim is kept. Any constant that file then defines is treated as this autoload's own result and parked on the Autoload; the constant table keeps UNDEF, autoload? and const_defined? report correctly, and every later lookup from another thread (Marshal.load, const_get, class reopening) goes through LoadService#featureAlreadyLoaded again instead of reading the table.

The feature is "already loaded or loading" in three common shapes: a direct require of the autoload's file, several constants declared with autoload against one file (ActiveSupport::Autoload's autoload_at, the usual Rails layout) where the first one loaded parks its siblings, and a circular require between two autoloaded files.

On the command line that walk is cheap. Under a classloader-backed $LOAD_PATH (a WAR) each lookup costs about 450 µs; a page deserialising 89 cached objects spent 1,229 ms in Marshal.load, 36 ms once the slots held their values.

The fix releases the claim in the already-loaded branch. The specs pin what Ruby can observe (a Marshal.load and a $LOAD_PATH change from another thread no longer depend on the loader; remove_const returns the value; every constant of a shared file becomes a regular constant); the test/jruby cases pin the constant-table slot itself, including the sibling and circular shapes.