Settle empty discovery searches and filename selections

docs/roadmap/discovery.md

@@ -260,6 +260,26 @@ on overlays with explicit context. Both ways of obtaining an absent main input
 have the same behavior. Implementation must cover this lifecycle change as well
 as candidate selection.
 
+### Empty Composition And Implicit Preset Selection (OD-4 — Settled)
+
+Require explicit preset or search selection to activate a convention. Supplying
+no presets or custom searches produces no candidates and activates no implicit
+fallback. Empty custom path lists and explicitly empty filename-group selections
+also contribute no candidates. An empty component does not prevent later
+components from contributing candidates.
+
+If the complete search is empty, an optional search follows the absent-main
+lifecycle settled under OD-3; a required search fails as exhausted. Empty lists
+are valid, supporting consumers that construct their searches programmatically.
+
+Distinguish omission from an explicitly empty filename selection for a directory
+search. Omitting the selection uses the agreed `[Named, Generic]` default;
+explicitly selecting an empty list means no filenames. The eventual API must
+represent both intentions clearly. Presets retain their specified filename groups.
+
+Selecting a single preset keeps the simple case concise. Search-wide requiredness
+provides the check for consumers that must find a configuration file.
+
 ### Placement, Provenance, And Existing Features
 
 The selected file participates as the actual file source. Preserve its physical
@@ -285,7 +305,8 @@ The subsequent implementation plan should cover:
 
 - Define filename-group, directory-search, preset, and search-source representations,
   slug validation, and a search-wide requiredness option. Keep construction
-  separate from acquisition and the simple preset selection concise.
+  separate from acquisition and the simple preset selection concise. Distinguish
+  omitted and explicitly empty directory-search filename selections.
 - Implement ordered expansion and concatenation for presets, filename groups,
   custom searches, and explicit file candidates, followed by first-match selection
   through existing file access. Resolve each source once per consumption before
@@ -298,6 +319,7 @@ The subsequent implementation plan should cover:
 - Exercise preset, directory, filename-group, and filename ordering, reversed
   group precedence, the Unix system preset's 14 candidates and named-only `/etc`
   search, custom-list composition, both filename extensions, no-match behavior,
+  empty components and searches, omitted versus empty filename selections,
   required searches, unavailable preset bases, errors that must stop fallback,
   repeated loads, and untouched explicit `File`
   behavior. Cover imports, overlays, underlays, defaults, CLI, source versions,
@@ -315,23 +337,6 @@ its in-progress changes are part of a fixed implementation baseline.
 
 ## Open Decisions
 
-### OD-4 — Empty Composition And Implicit Preset Selection
-
-**Recommendation:** require explicit preset or search selection to activate a
-convention. An empty search composition has no candidates: optional search contributes no
-source, and required search fails as exhausted. Empty custom lists, including
-empty filename-group selections, follow the same rule. This supports programmatic
-list construction and introduces no hidden
-system, user, or working-directory fallback. One explicitly selected preset
-still gives the simple application a short call.
-
-**Alternative:** let omission of searches select a documented default preset.
-This shortens the simplest call, but omission and an intentionally empty computed
-list would need distinguishable representations. It also requires choosing a
-default convention across platforms and application styles. Another alternative
-is to reject an empty composition as invalid; that catches accidental empty lists
-but adds a special case beyond the search-wide requiredness rule.
-
 ### OD-5 — Slugs And Custom Path Interpretation
 
 **Recommendation:** require a nonempty single-component slug when a selected