perf: copy-on-write for sentry_value_t clone

[jpnurmi]

Problem:

Both out-of-process handlers, crashpad and native, flush the global scope to __sentry-event whenever any scope properties change, to make it available for the handler process.

In sentry__scope_apply_to_event, the process of merging the scope into an event object is quite heavy. Passing references (event { tags: ref(scope->tags) }) is out of question, because that way any before_send modifications would leak to the scope and cause data loss. Therefore, we currently pass clones (event { tags: clone(scope->tags) }) instead. The problem is that the more data it has (tags, context, ...), the slower it gets.

Solution:

Pass (event { tags: cow(scope->tags) }) where the list of tags is scope->tags with refcount 2. Only and only if the before_send hook modifies it, the copy is detached so that the scope remains untouched.

Details:

Add copy-on-write (COW) semantics to sentry__value_clone() for list and object values. Instead of deep-copying items/pairs on clone, the new thing_t shares the underlying list_t/obj_t data via a data-level refcount. The actual copy is deferred to mutation time (thing_detach), making clone O(1) for flat containers.

Two-level refcounting:

• thing_t.refcount: how many sentry_value_t handles reference the thing
• list_t.refcount / obj_t.refcount: how many thing_ts share the data

Container children (nested lists/objects) are still recursively cloned to maintain value semantics at each nesting level.

pytest -v "tests/benchmark.py::test_benchmark_scope[crashpad-set_tag]":

1000x	Median before	Median after	Improvement
Windows	4.826 ms	1.843 ms	61.81%
macOS	0.988 ms	0.174 ms	82.39%
Linux	0.004 ms	0.003 ms	25.00%

Close: #1551

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