Add #1467 GC unsafe-zone auto-trigger coverage

[andrewtdiz]

Summary

• add regression coverage for automatic threshold GC while GC_UNSAFE_ZONES is active
• cover both direct auto-trigger checks and deferred root-lock flushes re-checking unsafe zones
• document the 0.5.1025: SIGSEGV every ~5 min on long-running Fastify+ws server (GC use-after-free; same root as #1425, harder failure mode) #1467 before/after: v0.5.1025 fails these assertions, current main passes

Fixes #1467.

Root cause / verification

• Affected release: v0.5.1025 (73ceb70) still has lifetime Fastify/ws unsafe zones and gc_check_trigger() ignores GC_UNSAFE_ZONES.
• Existing main after Fix GC unsafe-zone handling for server adapters #1429 blocks automatic/deferred GC when unsafe zones are active and removes stale server-lifetime unsafe zones.
• The new tests fail on v0.5.1025 with collection_count incrementing while unsafe zones are active, and pass on this branch.

Local verification

• cargo fmt --all
• git diff --check
• PERRY_GC_TRACE=1 PERRY_GC_DIAG=1 cargo test -p perry-runtime unsafe_zone --lib -- --nocapture
• PERRY_GC_FORCE_EVACUATE=1 PERRY_GC_VERIFY_EVACUATION=1 cargo test -p perry-ext-fastify gc_mutable_scanner_rewrites_registered_roots --lib -- --nocapture
• PERRY_GC_FORCE_EVACUATE=1 PERRY_GC_VERIFY_EVACUATION=1 cargo test -p perry-ext-http-server gc_mutable_scanner_rewrites_server_wrapper_and_request_response_roots --lib -- --nocapture
• PERRY_GC_FORCE_EVACUATE=1 PERRY_GC_VERIFY_EVACUATION=1 cargo test -p perry-ext-ws gc_mutable_scanner_rewrites_client_and_server_listener_roots --lib -- --nocapture
• cargo build -p perry
• PERRY_GC_TRACE=1 tests/test_issue_1425_gc_unsafe_zones.sh