docs(gc): add precise-tracing API redesign plan

[Flamki]

This PR adds a focused planning note for the GC redesign direction after removing the gc_allocator experiment path.

What this adds

• notes/precise_tracing_api_plan.md
  • precise-tracing API direction without root/reference-count dependence
  • explicit invariants (rooting, weak/ephemeron, finalization/resurrection, teardown)
  • phased execution plan (contract + prototype + gated integration)
  • validation and risk controls aligned with current tracker issues
• README link for discoverability

Why

Recent maintainer guidance points toward API-focused GC redesign and staged integration, while #54 removes the Collector: Allocator experiment. This note captures a practical plan that matches that direction and keeps scope clear.

Scope

• docs only
• no runtime/API behavior changes

Validation

• cargo fmt --all -- --check
• cargo test --workspace