Summary

Adds it::d4np::memorypool::PoolMemoryResource, a std::pmr::memory_resource Adapter that
binds one Pool behind the runtime std::pmr interface — so a single resource can back
any std::pmr container (via std::pmr::polymorphic_allocator) without the per-type
PoolAllocator<T> rebind. This is the "door left open" in ADR-0018. Header-only and purely
additive — the frozen C ABI and the existing C++ types are unchanged, so this is a SemVer
MINOR (v1.2.0 candidate). It opens roadmap Milestone 9.

Motivation

PoolAllocator<T> (ADR-0018) bakes the pool into a container's type; std::pmr chooses the
storage policy at run time by handing a container a memory_resource*. ADR-0018 recorded the
pmr resource as deferred, not rejected. Issue #107 / Milestone 9 opens it.

Changes

• pool_memory_resource.hpp — PoolMemoryResource : public std::pmr::memory_resource. A
  non-owning back-reference to a Pool plus a configurable upstream resource. Deterministic
  (bytes, alignment) routing (mirroring ADR-0018 §2): fitting requests come from the pool
  (throwing std::bad_alloc on exhaustion — never falling back, so do_deallocate routes
  deterministically); over-sized / over-aligned requests go upstream
  (std::pmr::get_default_resource() by default). do_is_equal by (pool, upstream) identity.
  Gated behind PBR_MEMORY_POOL_HAS_PMR (a __cpp_lib_memory_resource feature-test) so the
  header is a no-op where <memory_resource> is unavailable.
• ADR-0042 — records the decision (routing, non-owning shape, exhaustion-throws-not-falls-back,
  configurable upstream, availability gate) and the rejected alternatives; ADR index row added.
• Dedicated doctest pool_memory_resource_test — gated like the header; proves pool routing +
  exhaustion + reissue, upstream routing (via null_memory_resource), is_equal, and an
  end-to-end std::pmr::list / std::pmr::vector / polymorphic_allocator round-trip.
• Spec — §5.2 lists the type, §7 maps it to ADR-0042, §7.1 no longer defers it.
• ROADMAP — Milestone 9 created (ADR-0037) with this as item 9.1; feat(cpp): opt-in debug hardening — freed-block poisoning, canaries, free-list safe-linking #109/test(ci): add a coverage-guided fuzzing harness for the pool surface #108/perf(bench): add jemalloc/tcmalloc baselines and p99 tail-latency reporting #111 listed as 9.2–9.4.
• Doxyfile — predefines __cpp_lib_memory_resource so the gated class appears on the API site.
• CHANGELOG — [Unreleased] Added entry.

Design Patterns

• Adapter — a second realisation of the already-catalogued Adapter (ADR-0018), bridging the
  pool to the runtime std::pmr::memory_resource interface. No new pattern is introduced.

Verification

• MSVC build clean; pool_memory_resource doctest 5 cases / 33 assertions pass.
• clang-format (22.x) clean; clang-tidy clean on the header + test (the (bytes, alignment)
bugprone-easily-swappable-parameters findings are the fixed std::pmr override signature —
  suppressed with rationale).
• python tools/consistency_lint.py → OK.
• Full toolchain matrix / ASan / UBSan / TSan — via CI (the adapter runs under the existing
  test matrix; no threading added).

Documentation Impact

• ADR added (ADR-0042) + index row.
• Spec updated (§5.2, §7 map, §7.1).
• ROADMAP Milestone 9 created (ADR-0037).
• CHANGELOG [Unreleased] entry.
• PR metadata — assignee, feat label, v1.2.0 milestone.
• Deferred to the v1.2.0 release PR (to keep this feature PR off the translated docs
  surface — editing them trips the i18n-freshness gate): the README.md Milestone-9 table row
  (a release-time refresh per M8.8) and the docs/patterns/README.md Adapter-row refinement (the
  Adapter pattern is already catalogued; the pmr application is fully documented in ADR-0042 and
  the spec §7 map). The release PR refreshes both and re-syncs the zh-Hans / ja translations
  in one pass. The spec rows remain stale (already flagged since docs(spec): reconcile the memory-pool spec with the as-built system #110/docs(spec): add a C4 component diagram and record Mermaid as the diagram tooling #112).

Closes #107.