docs: add CLAUDE.md with project context (#1057)

Provide structured project context (overview, architecture, build/test,
key patterns, ecosystem position, dependencies, constraints) to enhance
development workflows.

Part of kcenon/common_system#560

Author: kcenon

CLAUDE.md

@@ -0,0 +1,98 @@
+# pacs_system
+
+## Overview
+
+Modern C++20 PACS (Picture Archiving and Communication System) implementation with
+DICOM standard implemented from scratch — zero external DICOM library dependency.
+Leverages the kcenon ecosystem for infrastructure (networking, threading, logging,
+containers). Currently in Phase 4 (Advanced Services & Production Hardening).
+
+## Architecture
+
+12 library targets organized in layered dependency:
+
+```
+pacs_core        - Tags, elements, datasets, Part 10 file I/O, dictionary
+pacs_encoding    - VR types, transfer syntaxes, compression codecs
+pacs_security    - RBAC, anonymization (PS3.15), digital signatures, TLS
+pacs_network     - PDU encode/decode, association state machine, DIMSE, pipeline
+pacs_client      - Job/routing/sync/prefetch/remote node management
+pacs_services    - SCP/SCU implementations, SOP class registry, IOD validators
+pacs_storage     - File storage, SQLite indexing, cloud (S3/Azure), HSM
+pacs_ai          - AI service connector, result handler (SR/SEG)
+pacs_monitoring  - Health checks, Prometheus metrics
+pacs_workflow    - Auto prefetch, task scheduler, study lock
+pacs_web         - REST API (Crow), DICOMweb (WADO/STOW/QIDO)
+pacs_integration - Adapters to kcenon ecosystem
+```
+
+Web frontend: `web/` — React + Vite + Tailwind + React Query (separate build).
+
+## Build & Test
+
+```bash
+# Quick build
+cmake -S . -B build && cmake --build build
+cd build && ctest --output-on-failure --timeout 120
+
+# Build script with options
+./scripts/build.sh --debug --tests
+```
+
+Key CMake options:
+- `PACS_BUILD_TESTS` (ON) — Catch2 v3.4.0 unit tests
+- `PACS_BUILD_EXAMPLES` (OFF) — 32 example programs
+- `PACS_BUILD_BENCHMARKS` (OFF), `PACS_BUILD_SAMPLES` (OFF)
+- `PACS_BUILD_STORAGE` (ON) — Requires SQLite3
+- `PACS_BUILD_CODECS` (ON) — JPEG, JPEG2000, JPEG-LS, HTJ2K, RLE
+- `PACS_WITH_OPENSSL` (ON), `PACS_WITH_REST_API` (ON)
+- `PACS_WITH_AWS_SDK` (OFF), `PACS_WITH_AZURE_SDK` (OFF)
+
+Presets: `default`, `debug`, `release`, `asan`, `tsan`, `ubsan`, `ci`, `vcpkg`
+
+CI: Multi-platform (Ubuntu GCC/Clang, macOS, Windows MSVC), coverage, static analysis,
+sanitizers, fuzzing, DCMTK interop testing, CVE scan, SBOM.
+
+## Key Patterns
+
+- **DICOM from scratch** — Full SOP class implementations without dcmtk/GDCM dependency
+- **SOP classes** — Verification (C-ECHO), CT/MR/US/XA/NM/PET/RT/SR/SEG/MG/CR/SC Storage,
+  Patient/Study Root Q/R, Modality Worklist, MPPS, Storage Commitment, Print, UPS, PIR, XDS-I.b
+- **Transfer syntaxes** — Implicit/Explicit VR, JPEG Baseline/Lossless, JPEG 2000,
+  JPEG-LS, RLE, HTJ2K, HEVC, Frame Deflate, JPEG XL
+- **Network pipeline** — PDU -> DIMSE -> Service execution -> Response encode pipeline architecture
+- **Association state machine** — Full DICOM Upper Layer association management
+- **Result type** — `kcenon::common::Result<T>` throughout
+
+## Ecosystem Position
+
+**Tier 5** — Highest-level application in the kcenon ecosystem.
+
+```
+common_system      (Tier 0) [required] — Result<T>, IExecutor
+container_system   (Tier 1) [required] — DICOM serialization
+thread_system      (Tier 1) [via network_system]
+logger_system      (Tier 2) [via network_system]
+monitoring_system  (Tier 2) [optional via network_system]
+network_system     (Tier 4) [required] — TCP/TLS, async operations
+database_system    (Tier 3) [optional]
+pacs_system        (Tier 5)  <-- this project
+```
+
+## Dependencies
+
+**Required ecosystem**: common_system, container_system, network_system (which pulls thread_system, logger_system)
+**Required native**: ICU (charset encoding), SQLite3 >= 3.45.1 (storage)
+**Optional codecs**: libjpeg-turbo, OpenJPEG, CharLS, libpng, OpenJPH (HTJ2K), OpenSSL >= 3.0
+**Fetched**: Catch2 v3.4.0, Crow v1.3.1 (REST API), ASIO 1.30.2
+
+## Known Constraints
+
+- Windows `std::min`/`std::max`: Must use `(std::min)(a, b)` parenthesization (Windows.h macros)
+- `std::jthread` unavailable on AppleClang; auto-fallback to `std::thread` with `USE_STD_JTHREAD`
+- libjpeg-turbo cannot be built via FetchContent (must be system package)
+- `PACS_USE_MOCK_S3` prohibited in Release builds (CMake FATAL_ERROR)
+- Linux static linking: circular dependency requires `--start-group/--end-group` linker flags
+- C++20 modules experimental (CMake 3.28+, Clang 16+/GCC 14+)
+- UWP/Xbox unsupported
+- Test framework: Catch2 (differs from ecosystem's Google Test convention)