feat: Epoch nursery allocator with @compact primitive

[gavlooth]

Summary

• Epoch nursery allocator: Splits HEAP into stable region + per-thread nursery. At epoch boundary, nursery resets in O(1) pointer rewinds. Leverages IC's cycle-freedom: after SNF, all nursery data except the result tree is dead — no tracing needed.
• @compact primitive: @compact(term) normalizes to SNF, deep-copies live result to stable region, resets nursery. Enables iterative algorithms (Bellman-Ford, etc.) that would otherwise exhaust the heap.
• 9 safety hardening fixes from production audit:
  1. CRITICAL: Nested @compact support via EPOCH_DEPTH counter — inner compacts don't corrupt outer epoch
  2. HIGH: Null sentinel guard (loc==0) in compact_deep_copy — prevents infinite spin-wait on HEAP[0]
  3. HIGH: COMPACT_MAX_DEPTH (64K) recursion limit — prevents stack overflow on deep SNF trees
  4. MEDIUM: epoch_init validates stable_fraction >= 2
  5. MEDIUM: epoch_reset_zero warns about spin-wait amplification
  6. MEDIUM: F_EQL_R ext-field stale pointer documented
  7. MEDIUM: epoch_bench calls thread_set_count(1) before running
  8. LOW: epoch_reset_stable() helper replaces direct STABLE_NEXT manipulation
  9. LOW: Removed dead EPOCH_NUM variable

New files

File	Lines	Purpose
clang/heap/epoch.c	354	Epoch infrastructure: init, begin/reset, stable alloc, stats, benchmarks
clang/heap/free.c	8	Stub for heap_free_reset() backward compatibility
clang/prim/fn/compact.c	122	@compact primitive with epoch-aware deep copy
clang/docs/EPOCH_ALLOCATOR.md	120	Architecture documentation

CLI

./main <file.hvm4> --epoch        # Enable epoch mode (stable = 1/4 HEAP)
./main <file.hvm4> --epoch=8      # Stable = 1/8 HEAP (larger nursery)
./main --epoch-bench              # Run allocation microbenchmarks

Test results

• Single-threaded: 190 PASS, 0 FAIL, 6 SKIP (4 intentional parse errors + 2 collapse-mode)
• Multi-threaded (-T4): 187 PASS, 0 FAIL, 6 SKIP, 2 flaky (pre-existing MT non-determinism)
• Zero regressions: All tests produce identical results with and without --epoch

Benchmark (--epoch-bench)

Nursery alloc: 192 M allocs/s (zero overhead vs current allocator)
Epoch reset:   1 ns/reset (O(threads) pointer rewind)
Stable alloc:  195 M allocs/s
Multi-epoch:   0.16 ms/epoch (100K nursery allocs + 1K stable promotions)

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