feat(cmake): install/export + pkg-config for find_package support + ADR-0028 (M7.4)

[danielPoloWork]

Summary

Closes ROADMAP item 7.4 — Phase 1 distribution (ADR-0004 §5). Adds CMake install/export rules and a pkg-config file so consumers can find_package(pbr_memory_pool CONFIG REQUIRED) and link pbr::memory_pool, and switches the release packaging to cmake --install so Release tarballs are complete, find_package-ready install trees.

Motivation

The target already declared $<INSTALL_INTERFACE:include> but there were no install() rules — the library could only be vendored. ADR-0004 §5 Phase 1 requires find_package support and installable Release artifacts. A latent bug reinforced this: release.yml hand-copied only 3 of the 7 public headers and shipped no package config, so the "install the artifacts from a Release" claim was untrue.

Changes

• ADR-0028 — records the layout: GNUInstallDirs destinations, one exported target name, relocatable config, SameMajorVersion compatibility, the install gate, and pkg-config (with its documented configure-time-prefix limitation).
• CMake install/export (CMakeLists.txt + cmake/*.in): PBR_MEMORY_POOL_INSTALL option (default PROJECT_IS_TOP_LEVEL) gates install(TARGETS … EXPORT), the full it/d4np/memorypool/ header tree, the package config + version file, the pkg-config .pc, and LICENSE.
• EXPORT_NAME memory_pool so the installed imported target is pbr::memory_pool — identical to the in-build alias. One target_link_libraries(app pbr::memory_pool) line works whether the consumer vendors via add_subdirectory / FetchContent or links an installed package.
• release.yml packages via cmake --install instead of hand-copying — fixes the missing-headers / missing-config bug; tarballs are now complete install trees.
• Docs: ROADMAP 7.4, ADR index, README Install and consume, CHANGELOG.

Design Patterns

• None — build-system / packaging work; no new GoF pattern. (The catalogued patterns are unaffected.)

Verification

• Builds cleanly on the full toolchain matrix — the install block runs at configure/install time only; no build-time or source impact. All four workflow YAMLs re-validated as parseable.
• Unit tests pass — no library code changed.
• clang-tidy clean on the diff — no C/C++ source changed (CMake + YAML + Markdown only).
• ASan + UBSan / Valgrind — N/A, no runtime change.
• Install + consumer verified end-to-end locally (MSVC 19.51 + Ninja): configure → build → cmake --install to a staging prefix; a separate project then did find_package(pbr_memory_pool CONFIG REQUIRED) + linked pbr::memory_pool, built, and ran (consumer OK: got=7 block_size=64). The installed targets file declares add_library(pbr::memory_pool STATIC IMPORTED); the .pc carries the correct prefix/libdir/version.
• markdownlint clean on changed docs; all relative links resolve.

Note: the release.yml change can only be exercised at the next tag push (M7.7). The cmake --install path it now uses was validated locally and produced exactly the expected tree (lib + full headers + config + targets + version + .pc + LICENSE).

Documentation Impact

• README.md updated — Install and consume section.
• ROADMAP.md checkbox flipped — 7.4.
• ADR added/updated — ADR-0028 + index row.
• docs/patterns/README.md updated — N/A.
• Spec updated — N/A.
• CHANGELOG.md updated — Added (M7.4) + Changed (M7.4).