Fix reference ownership when deleting hash values

[aspeer]

I have forked the zeroperl repo to support a WASM build of my WebDyne framework. During that build I saw repeated traps in the Cloudflare WASM provider which were traced to a reference count issue

This PR seeks to fix that issue.

zeroperl_hash_delete decrements the value returned by hv_delete(..., 0),
but Perl has already made that return value mortal. The extra decrement can
release a value still retained by another caller when the temporary scope drains.

Use hv_delete(..., G_DISCARD) so Perl owns deletion cleanup, and check key
existence beforehand to preserve the boolean result, including undef-valued keys.
The exported signature is unchanged.

Adds a native libperl regression using the production function extracted
verbatim. It checks retained values after temporary cleanup, exactly one DESTROY
per object, undef values, missing/repeated deletions and invalid arguments.

Validation on native Perl 5.44.0, macOS arm64:

• Untouched upstream: the retained-reference assertion fails after FREETMPS.
• Patched: 100 cycles pass, including retained-value and destructor checks.
• git diff --check passes.