feat(core): add observer API

[ptondereau]

Description

We add Zend Observer API bindings behind the observer feature flag for building profilers, tracers, and instrumentation tools.

The implementation provides a FcallObserver trait with three methods: should_observe to filter which functions to observe (result is cached by PHP), begin called at function entry, and end called at function exit even on exceptions. FcallInfo provides metadata including function name, class name, filename, line number, and whether the function is internal.

The observer uses a factory pattern with Send + Sync requirements for thread safety in both NTS and ZTS PHP builds. Registration happens via ModuleBuilder::fcall_observer during module setup.

[coveralls]

Pull Request Test Coverage Report for Build 21322610570

Details

• 4 of 69 (5.8%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.4%) to 34.609%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/builders/module.rs	1	7	14.29%
src/zend/observer.rs	3	62	4.84%

Totals
Change from base Build 21296895878:	-0.4%
Covered Lines:	1827
Relevant Lines:	5279

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💛 - Coveralls