fix(load): report the actual object-load status

[ss-o]

.zi-load-object always reported success. Every immediate load looked like it worked.

The defect

___retval+=$?
return __retval

__retval is assigned nowhere in the file. Zsh evaluates a bare name in return arithmetically, and an unset parameter is 0:

zsh -fc 'f(){ local ___retval=3; return __retval; }; f; print $?'   -> 0
zsh -fc 'f(){ local ___retval=3; return ___retval; }; f; print $?'  -> 3

The two sibling returns a few lines away use the correct three-underscore name, so this is a single missing underscore.

Why it mattered

The sole caller was already written correctly:

.zi-load-object ... $___id $___opt
integer ___last_retval=$?
___retval+=___last_retval
...
if (( ___turbo && ZI[HAVE_SCHEDULER] && 0 == ___last_retval )) {

It captures the result, accumulates it once, and gates turbo scheduling on it. A constant 0 defeated all three: a failed immediate load was counted as success, and turbo scheduling proceeded anyway.

The trap in fixing it

Returning the real status while leaving ___retval+=$? in place would have double-counted every failure, because the helper was also adding to the caller's dynamically scoped accumulator. The fix separates the two: the helper gets its own local result and returns it, and the accumulation is dropped so the caller owns it, which is the contract the caller already assumed.

Test

tests/load-object-status.zsh stubs .zi-load and .zi-load-snippet so the status under test is unambiguously the one supplied, with no dependency on a real plug-in, the network, or the filesystem. It covers plugin and snippet across success and failure, and pins that the helper leaves the caller's $___retval untouched.

Observed against the unfixed helper:

plugin failure: expected 7, got 0
not ok - .zi-load-object does not report the status of the load it performed

Registered in zsh-n.yml (both path filters and a job), since that workflow enumerates tests explicitly and an unregistered file never runs.

Full suite 19/19, zsh -n clean.

This is item 1 of the Wave 1 list in docs/maintainers/core-architecture-audit.md, preserved on the docs/core-architecture-audit branch.

Closes #446