feat(chaos): audit trail for injection events (#44)

[mvillmow]

Summary

Closes #44. Adds ChaosAuditLog, a header-only structured JSON-lines audit trail that records every chaos fault injection and removal so chaos test outcomes are no longer ephemeral.

Each record captures:

• timestamp — ISO-8601 UTC with millisecond precision
• action — inject | remove
• fault_type — network-partition, latency, kill, queue-starve, ...
• fault_id — id returned by Agamemnon
• target — Agamemnon base URL the fault was sent to
• status — HTTP status returned by Agamemnon (0 on transport failure)
• requester — CHAOS_AUDIT_REQUESTER env, falling back to $USER, then unknown
• details — verbatim Agamemnon response body
• schema_version — 1

Configuration

Env var	Default	Purpose
CHAOS_AUDIT_LOG	stderr	File path for the audit trail. -, stderr, or unset emit to stderr. Any other value is appended to that file path.
CHAOS_AUDIT_REQUESTER	$USER or unknown	Identity recorded in each record's requester field.

If the requested file path cannot be opened the class falls back to stderr with a one-line warning rather than crashing the test process — audit emission is best-effort telemetry and must never break a chaos test. Writes are mutex-serialised, so one instance is safe across threads.

Why a file-based JSONL log (not NATS)?

The issue body asks for persistence "to stderr or a dedicated file". A header-only JSONL emitter:

• Captures the audit gap immediately (timestamp + environment + fault) with zero new infrastructure.
• Plays nicely with the existing hi.logs.> Promtail pipeline by tailing the file (or capturing stderr from the test runner) — no nats.c FetchContent needed.
• Stays well inside the MEDIUM scope cap (6 files, ~340 LOC added).

A future ticket can layer NATS publishing on top by reusing the same envelope.

Changes

• include/projectcharybdis/chaos_audit.hpp — new header-only ChaosAuditLog.
• test/src/test_chaos_audit_unit.cpp — 6 unit tests covering envelope shape, inject vs remove, multi-line ordering, and stderr-fallback paths.
• test/src/test_chaos_api.cpp — E01–E04 routed through inject() / remove_fault() helpers that emit audit records around every Agamemnon API call.
• cmake/SourcesAndHeaders.cmake, test/CMakeLists.txt — wire the new header and unit test into the build.
• README.md — documents CHAOS_AUDIT_LOG and CHAOS_AUDIT_REQUESTER.

Test plan

• Local build with conan + cmake (clang-tidy/cppcheck disabled locally due to pre-existing stddef.h toolchain issue on this host, unrelated to this PR — affects unchanged src/http_test_client.cpp on main too): ProjectCharybdis_tests and ProjectCharybdis_integration_tests both link clean.
• All 6 new unit tests pass.
• No regression in the 40 pre-existing unit tests (the one failing test, HttpTestClientUnit.MalformedUrlFallsBackToDefaults, fails on main too on hosts where something happens to listen on localhost:8080).
• CI: full cmake --preset ci build with -Werror, clang-tidy, cppcheck.

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