…e) (#113)

## 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;
#109/#108/#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

- [x] MSVC build clean; `pool_memory_resource` doctest **5 cases / 33
assertions pass**.
- [x] `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).
- [x] `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

- [x] ADR added (ADR-0042) + index row.
- [x] Spec updated (§5.2, §7 map, §7.1).
- [x] ROADMAP Milestone 9 created (ADR-0037).
- [x] CHANGELOG `[Unreleased]` entry.
- [x] 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
#110/#112).

Closes #107.

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>