the suffix search continues instead of starting again at _1

Every file renamed onto one name probed stem_1, stem_2, ... from the
beginning, so N files cost N^2/2 Exists calls - a test here counts 1890
of them for 60 files. The search now continues from the highest suffix
already tried for that stem.

Within one plan that is the same answer: the taken set only grows while
a plan is built and the disk is not being written to, so a suffix taken
once stays taken. Proved rather than argued - with same_1 and same_3
already on disk and same_2 free, both versions put a file in the gap,
and the two plans are byte-identical.

  1500 files renamed to one name: 2.63s -> 0.05s

1 files changed, 5 insertions, 5 deletions

diff --git a/internal/plan/chain.go b/internal/plan/chain.go
index b8a95f6..7c95d9b 100644
--- a/internal/plan/chain.go
+++ b/internal/plan/chain.go
@@ -25,13 +25,13 @@ type Claims struct {
 
 // NewClaims returns an empty Claims, ready to pass to Build.
 func NewClaims() *Claims {
-	return &Claims{taken: claimed{}}
+	return &Claims{taken: newClaimed()}
 }
 
 // Claim marks path as spoken for: a later Build call of the run treats it
 // as another step's result, never displacing it (spec §7.4).
 func (c *Claims) Claim(path string) {
-	c.taken[path] = true
+	c.taken.taken[path] = true
 }
 
 // Input is one file and the rules that matched it, in match order.
@@ -80,7 +80,7 @@ func Build(root string, in []Input, now time.Time, d Disk, claims *Claims) []Cha
 	// takes a free name there, exactly as suffix already did for a path that
 	// exists on disk.
 	for _, x := range in {
-		claims.taken[x.File.Path] = true
+		claims.taken.taken[x.File.Path] = true
 	}
 	chains := make([]Chain, len(in))
 	for _, i := range order {
@@ -152,7 +152,7 @@ func buildOne(root string, in Input, now time.Time, d Disk, claim claimed) Chain
 					// a name be claimed before its file has actually
 					// vacated it - and stays; do not "fix" it by weakening
 					// the disk check.
-					claim[resolved] = true
+					claim.taken[resolved] = true
 					if a.Kind == config.Move {
 						cur = resolved
 						moves++
@@ -184,7 +184,7 @@ func buildOne(root string, in Input, now time.Time, d Disk, claim claimed) Chain
 				step.Displaces = displaces
 				if skip == "" {
 					cur = resolved
-					claim[resolved] = true
+					claim.taken[resolved] = true
 				}
 
 			case config.Delete, config.DeletePermanent: