Restructure DEVELOPMENT.md to match internal SDK documentation template

[Copilot]

Rewrites DEVELOPMENT.md from a contributor how-to into an internal technical reference aligned with the standard Bugsnag SDK documentation structure.

Structure changes

• Replaces repo-layout/build-system/CI sections with template-driven sections: Initialization, Error Handling, Stacktrace Capture, Handled Events, Callbacks, Breadcrumbs, Metadata, Feature Flags, Launch Crashes, Network Errors, Session Tracking, Payload Serialisation, Delivery and Persistence, Symbolication, Related SDKs
• [Performance] sections omitted (error-only SDK)

Content added

• Initialization: Start() sequence, BugsnagAutoInit mechanism, SDK state before start, what runs on background threads
• Error handling: per-type breakdown — BugsnagLogHandler for C# exceptions, Android JVM/NDK, iOS/macOS native layer, app hangs, ANRs
• Stacktrace capture: C# regex parsing formats, IL2CPP il2cpp_native_stack_trace() P/Invoke, SendThreads/MaxReportedThreads behaviour
• Callbacks: OnError/OnSendError/OnSession — storage, invocation order, silent exception swallowing, .ToList() copy-before-iterate pattern
• Breadcrumbs: LinkedList FIFO storage, all automatic trigger types, native forwarding caveat
• Metadata: dictionary-of-dictionaries structure, reference-merge concurrency caveat (by-design, avoid concurrent mutation with Notify()), key redaction
• Feature flags: OrderedDictionary storage, snapshot copy taken at event-build time
• Delivery and persistence: full flow, HTTP status → action table, disk cache limits (Events/ 32 max, Sessions/ 128 max, 60-day TTL), 1 MB truncation algorithm
• Symbolication: Android .so + IL2CPP mapping upload, iOS/macOS dSYM injection into Xcode build phase, Bugsnag CLI auto-download, build identifiers used per platform
• Configuration options: table of non-obvious/platform-specific options (ReportExceptionLogsAsHandled, SendLaunchCrashesSynchronously, AppHangThresholdMillis, MaximumTypePerTimePeriod as Dictionary<LogType, int>, etc.)

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