[SDK-6c] Extension point: IParameterSource

[ChrisonSimtian]

Expose IParameterSource as a public extension point for [Parameter] value resolution. Maps to surface 3 in RFC #2.

Scope

• Plugins resolve parameter values from custom sources (Vault, 1Password, env, args, parameter files).
• Refactor ParameterService reflection logic into the chain-of-sources model.
• Define resolution order and conflict semantics (SDK-4 hook).

Depends on

SDK-1, v11 #92 (FalloutBuild split).