feat(bootstrap): OnConfigLoaded hook to inspect and override loaded config

[tgiachi]

Summary

The config manager loads YAML transparently - you could not see the values nor tweak them at startup. New typed hook on the bootstrap:

bootstrap.OnConfigLoaded<SquidStdLoggerOptions>(o => o.MinimumLevel = LogLevelType.Debug);
bootstrap.OnConfigLoaded<WorkersConfig>(w => w.MaxConcurrency = 8);

• Runs after every configuration load and before the logger and services consume the sections. Mechanism: a new IConfigManagerService.ConfigLoaded event raised at the end of Load() - this makes hooks survive the second Load that the config manager's own StartAsync performs during startup (fresh instances would otherwise wipe the overrides).
• In-memory only: the YAML file is never rewritten (Save() stays explicit). Default behavior unchanged with zero hooks.
• Hooks compose in registration order; registering for a non-section type fails at startup with a clear error; registering after start throws (same state guard as ConfigureServices).
• Visibility documented too: Compose() (YAML dump of current values) and GetConfig<T>() in the configuration guide, with three verified examples; lifecycle concept updated (load -> hooks -> logger -> services); tutorial cross-link; Services.Core README snippet.

Test Plan

• 7 new tests incl. OnConfigLoaded_SurvivesTheSecondLoad (counter >= 2 across the double Load) and OnConfigLoaded_DoesNotRewriteYamlFile (disk vs memory)
• Full suite green (known SQS load-flake passed isolated); build 0 errors, 0 new warnings; docfx 0 warnings, anchors verified
• No BREAKING markers (expected release: 0.18.0)

Follow-ups noted (non-blocking)

• Retry after a failed second Load leaves un-hooked section instances (bootstrap retry semantics are already rough - pre-existing)
• ApplyConfigHooks could guard _disposed for externally-owned containers