Phase 10.4d — Barrier Strict Mode (CI/Local)

Goal
- Catch missing Write-Barrier quickly in development and CI.

How-To (Local)
1) Build with default features:
   cargo build --release -j32
2) Run with CountingGc + Strict Barrier + Trace:
   (in code) build runtime via NyashRuntimeBuilder::with_counting_gc()
   (env)
   set NYASH_GC_BARRIER_STRICT=1
   set NYASH_GC_TRACE=1
   nyash <program.hako>

Expected
- Barrier sites log to stderr.
- On any mutating site without barrier increment, process panics with site name.

CI Suggestion
- Add a job that runs selected examples/tests with CountingGc enabled and
  NYASH_GC_BARRIER_STRICT=1. Fail build on panic.

Notes
- Strict mode requires CountingGc; otherwise the helper panics to avoid false negatives.
- Use NYASH_GC_TRACE=2 for detailed roots breakdown at end-of-run when investigating.

