fix: Activity action/eligibility callback ABI (dropped AnalysisContext)

[tinysec]

Summary

The C core invokes an Activity's action as void(*)(void*, BNAnalysisContext*) and its eligibility as bool(*)(void*, BNActivity*, BNAnalysisContext*) (workflow.py). The binding marshaled both as Action<IntPtr> (one argument, void return), so:

• the action never received its AnalysisContext — the entire point of the callback;
• the eligibility handler received neither handle and returned garbage bool (reading whatever was in the return register) — eligibility was effectively random.

This made the workflow Activity API (the foundation of BN's analysis-extension model) unusable.

Changes

• New public delegates ActivityAction(AnalysisContext) and ActivityEligibility(Activity, AnalysisContext), plus native-shaped ActivityActionDelegate / ActivityEligibilityDelegate and holder contexts that wrap the raw handles via AnalysisContext/Activity.NewFromHandle (borrowed handles; the wrapper owns the reference it adds and frees it on dispose). Mirrors the established MatchDataDelegate pattern.
• Activity.Create / CreateWithEligibility now wrap the user delegates and root the wrappers (not the user delegates) on the instance — the wrapper's function pointer is what the core invokes.
• BNCreateActivity / BNCreateActivityWithEligibility P/Invoke signatures are unchanged (config, ctxt, action[, eligibility]); only the delegate marshaling was wrong.

Verification

• Binding builds clean (0 warnings, 0 errors).
• New E2E test builds an Activity with the corrected delegates and registers it in a cloned editable workflow — exercising the real P/Invoke path (GetFunctionPointerForDelegate over the new delegate types, BNCreateActivityWithEligibility, BNWorkflowRegisterActivity) against the core.
• Full harness regression: 39/39 pass (no regressions).

Coverage note (honest)

The callback is only invoked when the core runs the activity during analysis, which requires opening a view under a custom workflow — a workflow-machine surface (WorkflowMachine, view-creation-under-custom-workflow) not yet bound. The function workflow DAG is immutable and there is no switch-workflow request command, so firing the callback end-to-end is deferred to that binding. The delegate shapes here mirror Python's ctypes.CFUNCTYPE declarations exactly, which are the authoritative ABI spec.