Add Config singleton cache (scoped first cut of #8)

[ff225]

Summary

Partial fix for #8 — scoped to avoid conflicting with the in-flight PRs that touch request_handler.py, http_server.py, and message_data.py.

Investigated the current state first: src/sciot/config.py already has solid YAML loading, env-var overrides, and full schema validation — the "prerequisite for modularity" work the issue's own comment worried about is largely done. The real gap was just caching/access: every call site independently called load_server_config/load_client_config and re-parsed + re-validated the YAML on every call, except request_handler.py's own ad-hoc _settings_cache.

Added src/common/config.py::Config, a small singleton-style cache with Config.get_server(path=None) / Config.get_client(path=None) wrapping the existing sciot.config loaders, caching per resolved path. Migrated three call sites that don't touch the in-flight-PR files:

• src/server/edge/edge_initialization.py (load_delay_config, __main__ block)
• src/server/logger/log.py (configure_logger_from_settings)
• src/server/edge/run_edge.py (main)

Scope notes

Left untouched on purpose:

• src/server/communication/request_handler.py — already has its own _settings_cache, and is being actively modified by PR Add device/edge CPU, network latency, switch penalty to offloading_decisions #68/Fix offloading_decision_summary.py to read all split segments #69.
• src/client/python/http_client.py — loads client config eagerly at import time with its own CONFIG_PATH env override; migrating that is riskier and out of scope for this first cut.
• src/sciot/cli.py — one-shot config-validate invocations, no repeated-read problem to fix.

A follow-up can migrate request_handler.py/http_client.py to Config once the in-flight PRs land, and/or fold sciot.config's standalone functions behind Config entirely if that's the desired end state — leaving that as a separate, smaller PR rather than bundling it here.

Test plan

• Added tests/unit/test_common_config.py: per-path caching (single parse across repeated calls), force_reload, reset(), independent caching across different paths, default-path resolution for both server and client configs.
• uv run pytest tests/unit/test_common_config.py -q — 8 passed
• Fast CI suite (tests/unit, integration subset, mqtt, offloading_algo) — 211 passed, 1 skipped, no regressions
• uv run ruff check on all changed files — clean

Related: closed #3 separately (stale — the "redundant JSON parsing for offloading history" it described was already fixed by existing class-level caches in offloading_algo.py/request_handler.py).