Undeclared-placeholder scan ignores --customize exclusions, prompting for deselected components

[bguidolim]

Problem

ConfiguratorSupport.scanForUndeclaredPlaceholders (Sources/mcs/Sync/ConfiguratorSupport.swift:358-412) walks pack.components with no awareness of --customize exclusions:

for pack in packs {
    for component in pack.components {
        switch component.installAction {
        case let .copyPackFile(source, _, _): ...
        case let .settingsMerge(source): ...
        case let .mcpServer(config): ...

So a placeholder that appears only inside a deselected component's files is still collected, and Configurator.swift:692-706 prompts for it:

Set value for SOME_KEY

…for a file that will never be installed. The answer is stored in state.resolvedValues and substituted nowhere.

Configurator.configure already has excludedComponents in hand (Configurator.swift:228) and threads it into auto-install, template preloading, artifact installation, and settings composition. resolveAllValues (:642) is the one step that never received it.

Fix sketch

Thread excludedComponents from configure() → resolveAllValues → scanForUndeclaredPlaceholders, and filter inside the component loop:

for pack in packs {
    let excluded = excludedComponents[pack.identifier] ?? []
    for component in pack.components where !excluded.contains(component.id) {

Templates (includeTemplates: true, global scope) stay unfiltered — template dependency filtering is a separate mechanism handled by preloadTemplates.

Care required

This scan is load-bearing: it is the safety net that catches placeholders which would otherwise survive emitWarnings: false substitution and get written literally into installed files. Narrowing it must not create a hole for a selected component:

• Filter strictly by component id — do not narrow by artifact family or source path.
• Excluded-but-required components (ComponentDefinition.isRequired) can never appear in the exclusion set (ConfiguratorSupport.selectComponentExclusions:140-141), so they stay covered.

Tests

Tests/MCSTests/LifecycleIntegrationTests.swift (or ConfiguratorSupport-level unit tests):

• placeholder only in an excluded component's copyPackFile source → not prompted
• same placeholder also used by a selected component → still prompted (regression guard)
• excluded settingsMerge source and excluded mcpServer env/command/args → not prompted (all three families the scan covers)
• no exclusions → unchanged behavior

Relation to #356

#356 proposes requiredBy on prompts, and correctly identifies that narrowing this scan is a prerequisite — otherwise a requiredBy-skipped prompt reappears as a raw Set value for X fallback. This half is independently valuable and carries no manifest schema change, so it is being split out.