feat(Config): Implement configuration severity policy and logging mechanism (#23284)

doc/ConfigPolicy.md

@@ -0,0 +1,101 @@
+# Configuration Severity Policy
+
+The configuration loader can decide how strictly it should react when it
+encounters missing files, undefined options or invalid values. This document
+describes the available knobs and provides ready-to-use presets.
+
+## Severity Levels
+
+Each policy entry maps a **key** to one of the following severities:
+
+| Severity | Description                                                                 |
+|----------|-----------------------------------------------------------------------------|
+| `skip`   | Ignore the problem and continue silently.                                   |
+| `warn`   | Log a warning and continue.                                                 |
+| `error`  | Log an error and continue (useful to surface issues without aborting).      |
+| `fatal`  | Log a fatal message and abort the process immediately.                      |
+
+## Policy Keys
+
+The following keys can be customised:
+
+| Key                | Applies to                                                            |
+|--------------------|----------------------------------------------------------------------|
+| `default`          | Fallback severity for any key that is not explicitly overridden.     |
+| `missing_file`     | Missing or empty configuration files (worldserver.conf, modules, …). |
+| `missing_option`   | Options looked up in code but not present in any config file.         |
+| `critical_option`  | Required options (`RealmID`, `*DatabaseInfo`, …).                     |
+| `unknown_option`   | Options found in optional configs that the core does not recognise.   |
+| `value_error`      | Options that cannot be converted to the expected type.                |
+
+> Critical options remain fatal by default to prevent the core from booting with
+> incomplete database details; you can relax them if required.
+
+## Configuration Channels
+
+### `config.sh`
+
+`conf/dist/config.sh` exposes the `AC_CONFIG_POLICY` variable alongside a few
+presets:
+
+```bash
+# Mirrors the default behaviour (errors, with fatal criticals)
+export AC_CONFIG_POLICY="$AC_CONFIG_POLICY_PRESET_DEFAULT"
+
+# Skip anything non-critical so the core can bootstrap from defaults + env vars
+export AC_CONFIG_POLICY="$AC_CONFIG_POLICY_PRESET_ZERO_CONF"
+
+# Treat everything strictly (useful for CI)
+export AC_CONFIG_POLICY="$AC_CONFIG_POLICY_PRESET_STRICT"
+```
+
+The presets are defined as:
+
+```bash
+AC_CONFIG_POLICY_PRESET_DEFAULT='missing_file=error,missing_option=warn,critical_option=fatal,unknown_option=error,value_error=error'
+AC_CONFIG_POLICY_PRESET_ZERO_CONF='default=skip,critical_option=fatal,unknown_option=warn,value_error=warn'
+AC_CONFIG_POLICY_PRESET_STRICT='default=error,missing_file=fatal,missing_option=error,critical_option=fatal,unknown_option=error,value_error=error'
+```
+
+Modify or extend these entries to suit your deployment.
+
+### Environment Variable
+
+The runtime honours the `AC_CONFIG_POLICY` environment variable, so you can
+override the policy without editing `config.sh`:
+
+```bash
+export AC_CONFIG_POLICY="default=skip,critical_option=fatal"
+./acore.sh run-worldserver
+```
+
+### CLI Override
+
+Every server/tool executable accepts `--config-policy`:
+
+```bash
+./bin/worldserver --config-policy="missing_file=fatal,unknown_option=warn"
+./bin/authserver --config-policy "$AC_CONFIG_POLICY_PRESET_STRICT"
+```
+
+The CLI flag takes precedence over the environment and `config.sh`.
+
+## Quick Presets
+
+| Preset        | Intended use                                                              |
+|---------------|---------------------------------------------------------------------------|
+| `legacy`      | Default behaviour before this feature (errors for missing files/options). |
+| `zero-conf`   | Zero-touch deployments; rely on defaults/env vars where possible.         |
+| `strict`      | Fail-fast in CI or controlled environments.                               |
+
+Feel free to clone these presets and store your own variants inside
+`config.sh` or deployment scripts.
+
+## Tips
+
+- Pair `fatal` severities with monitoring so regressions in configuration
+  surface quickly.
+- When experimenting locally, start with `zero-conf` and elevate specific keys
+  to `error`/`fatal` as you validate your setup.
+- Remember that number parsing errors (`value_error`) often indicate typos;
+  keep them at least `error` unless you have a very good reason.