Fix xref cleanup ordering and purge

[0verflowme]

Summary

• free the xref manager before destroying the analysis lock
• reset xrefs when purging an analysis context
• verify purge clears old references and leaves the manager reusable

Root cause

r_anal_free() destroyed anal->lock before calling r_anal_xrefs_free(), even though xref cleanup acquires that lock. This could access a destroyed mutex during teardown.

r_anal_purge() reset functions, metadata, types, and imports but retained the old xref manager, allowing references from the previous analysis to survive into the next one.

Validation

• clean build from current upstream master
• test_anal_xrefs: 2/2 passing
• git diff --check