refactor(chunker): drop redundant TierRecursive

TierRecursive and TierLegacy both invoked SplitText with identical
output, but only TierLegacy got the "always-return-on-validation-failure"
safety-net behavior. Auto chains thus ran SplitText twice on the
fallback path and produced two-line debug rejection traces with the same
reason.

Inline TierRecursive into TierLegacy: SelectStrategy and the explicit
StrategyRecursive entry point now emit single-legacy chains. The
StrategyRecursive public constant stays so existing ChunkingConfig rows
keep parsing — it's just an alias for legacy now.

No user-visible behavior change; one fewer SplitText call per failed
auto fallback.

internal/infrastructure/chunker/profiler.go

@@ -210,7 +210,6 @@ type StrategyTier string
 const (
 	TierHeading   StrategyTier = "heading"
 	TierHeuristic StrategyTier = "heuristic"
-	TierRecursive StrategyTier = "recursive"
 	TierLegacy    StrategyTier = "legacy"
 )
 
@@ -220,7 +219,7 @@ const (
 // a final safety net so callers always receive at least one chunk-set.
 func SelectStrategy(p *DocProfile) []StrategyTier {
 	if p == nil {
-		return []StrategyTier{TierRecursive, TierLegacy}
+		return []StrategyTier{TierLegacy}
 	}
 	var chain []StrategyTier
 
@@ -235,7 +234,8 @@ func SelectStrategy(p *DocProfile) []StrategyTier {
 		chain = append(chain, TierHeuristic)
 	}
 
-	// Always end with recursive (Tier 3) and then legacy as ultimate fallback.
-	chain = append(chain, TierRecursive, TierLegacy)
+	// Legacy is the ultimate fallback: always returns chunks even when
+	// validation fails, so callers never get an empty result.
+	chain = append(chain, TierLegacy)
 	return chain
 }

internal/infrastructure/chunker/profiler_test.go

@@ -144,8 +144,8 @@ func TestSelectStrategy_PlainDoc(t *testing.T) {
 	doc := "just a paragraph of plain text without any structure indicators at all here"
 	p := ProfileDocument(doc)
 	chain := SelectStrategy(p)
-	if chain[0] != TierRecursive {
-		t.Errorf("expected recursive tier first for unstructured doc, got %v", chain)
+	if chain[0] != TierLegacy {
+		t.Errorf("expected legacy tier first for unstructured doc, got %v", chain)
 	}
 }
 

internal/infrastructure/chunker/strategy.go

@@ -185,7 +185,9 @@ func resolveChainWithProfile(text string, cfg SplitterConfig) ([]StrategyTier, *
 	case StrategyHeuristic:
 		return []StrategyTier{TierHeuristic, TierLegacy}, nil
 	case StrategyRecursive:
-		return []StrategyTier{TierRecursive, TierLegacy}, nil
+		// "recursive" is a public-API alias for "legacy": both invoke
+		// SplitText. Kept for backwards compatibility with stored configs.
+		return []StrategyTier{TierLegacy}, nil
 	case StrategyLegacy, "":
 		// Empty == legacy preserves backwards compatibility with stored
 		// ChunkingConfig rows that pre-date the Strategy field.
@@ -200,16 +202,16 @@ func resolveChainWithProfile(text string, cfg SplitterConfig) ([]StrategyTier, *
 
 // runTier dispatches the splitter implementation for the given tier.
 // splitByHeadings / splitByHeuristics are package-level vars overridden
-// from heading_splitter.go / heuristic_splitter.go via init(); recursive
-// and legacy share the same SplitText path. The default branch is kept
-// as defensive belt-and-suspenders for future StrategyTier additions.
+// from heading_splitter.go / heuristic_splitter.go via init(); legacy
+// runs SplitText. The default branch is defensive for future
+// StrategyTier additions.
 func runTier(tier StrategyTier, text string, cfg SplitterConfig) []Chunk {
 	switch tier {
 	case TierHeading:
 		return splitByHeadings(text, cfg)
 	case TierHeuristic:
 		return splitByHeuristics(text, cfg)
-	case TierRecursive, TierLegacy:
+	case TierLegacy:
 		return SplitText(text, cfg)
 	}
 	return SplitText(text, cfg)