WIP: perf(crashpad): replace scope flushing with incremental IPC updates

[jpnurmi]

Warning

WIP    🚧🔨⏳⛔

The Crashpad backend now writes a small initial __sentry-event and mirrors scope changes to the Crashpad handler as MessagePack updates. This avoids rewriting the full event and external crash report on every scope mutation, which is especially expensive on Windows.

Function	Iterations	Before* (ms)	After* (ms)
sentry_set_tag	1000	5.839 +/- 4.564 [1.445..49.623]	0.135 +/- 0.032 [0.074..0.319]
sentry_add_breadcrumb	1000	0.365 +/- 1.243 [0.183..39.475]	0.119 +/- 0.028 [0.052..0.253]

*) avg +/- stddev [min..max]

Breadcrumbs and attachments are sent through the new scope observer path while preserving the existing breadcrumb rotation. Crash events are still assembled as __sentry-event; Crashpad merges the accumulated handler-side scope state when the report is written.

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## Unreleased

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- replace scope flushing with incremental IPC updates ([#1840](https://github.com/getsentry/sentry-native/pull/1840))

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