feat: skip existing pages on re-run, add --story-slug for targeted repairv0.18.0

- loadOrGenerate() in artist.go skips API calls for pages already on disk,
  loading bytes for the reference chain. Re-runs now only generate missing
  pages instead of regenerating the whole comic.
- --story-slug flag forces the output directory slug so a repair run writes
  to the same directory as the original (e.g. --story-slug ai-jungle-quest).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

5 files changed, 50 insertions, 9 deletions

diff --git a/internal/cli/command.go b/internal/cli/command.go
index a1168b5..090b2d1 100644
--- a/internal/cli/command.go
+++ b/internal/cli/command.go
@@ -69,6 +69,9 @@ func setupFlags(cmd *cobra.Command, flags *Flags) {
 	cmd.Flags().StringVar(&flags.StoryStyle, "story-style", "", "Art style for comic pages (default: random). E.g. \"ultra realistic comic strip with photographic detail and dramatic lighting\"")
 	cmd.Flags().StringVar(&flags.StoryTheme, "story-theme", "", "Genre/theme for the story (default: random). E.g. \"a thrilling space adventure with aliens and spaceships\"")
 	cmd.Flags().BoolVar(&flags.StoryNoUltraRealistic, "no-ultra-realistic", false, "Disable photorealistic rendering requirement; produces standard comic-book style output")
+	cmd.Flags().StringVar(&flags.StorySlug, "story-slug", "",
+		"Force the output directory slug for --story (e.g. \"ai-jungle-quest\"). "+
+			"Use this to repair a partial run: existing pages are skipped, missing ones are generated.")
 	cmd.Flags().StringVar(&flags.NarratorVoice, "narrator-voice", "",
 		"Gemini voice for cinematic story narration (default: random from cinematic pool). "+
 			"Valid values: Charon, Fenrir, Enceladus, Algieba, Aoede, Schedar")
diff --git a/internal/cli/flags.go b/internal/cli/flags.go
index 104f721..9c83138 100644
--- a/internal/cli/flags.go
+++ b/internal/cli/flags.go
@@ -23,7 +23,8 @@ type Flags struct {
 	StoryFile            string // --story <file>: generate vocabulary story + comic image
 	StoryStyle           string // --story-style: override the random art style (empty = random)
 	StoryTheme           string // --story-theme: override the random genre pick (empty = random)
-	StoryNoUltraRealistic bool  // --no-ultra-realistic: disable photorealistic rendering requirement
+	StoryNoUltraRealistic bool   // --no-ultra-realistic: disable photorealistic rendering requirement
+	StorySlug            string // --story-slug: force a specific output slug/directory (empty = auto from title)
 	NarratorVoice        string // --narrator-voice: Gemini voice for cinematic narration (empty = random)
 	SkipAudio         bool
 	SkipImages        bool
diff --git a/internal/story/artist.go b/internal/story/artist.go
index 885717a..4f9ff4c 100644
--- a/internal/story/artist.go
+++ b/internal/story/artist.go
@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"math/rand/v2"
 	"os"
+	"path/filepath"
 	"strings"
 	"time"
 
@@ -217,7 +218,9 @@ func (a *Artist) DrawComicPages(storyText, prebuiltBible, titleSlug string, entr
 	// 1. Cover — generated without refs (it is the visual baseline).
 	// Retried up to pageMaxRetries times; failure is non-fatal but the cover
 	// is omitted from the PDF and no anchor reference is established.
-	p, coverBytes := a.generatePageWithRetry(buildCoverPrompt(storyText, style, bible, a.renderReq()), titleSlug+"_cover", nil, "cover page")
+	p, coverBytes := a.loadOrGenerate(titleSlug+"_cover", func() (string, []byte) {
+		return a.generatePageWithRetry(buildCoverPrompt(storyText, style, bible, a.renderReq()), titleSlug+"_cover", nil, "cover page")
+	})
 	if p != "" {
 		paths = append(paths, p)
 		recentRefs = appendRef(recentRefs, coverBytes) // cover becomes the anchor reference
@@ -232,7 +235,9 @@ func (a *Artist) DrawComicPages(storyText, prebuiltBible, titleSlug string, entr
 		pageNum := i + 1
 		prompt := buildStoryPagePrompt(section, pageNum, storyPageCount, style, bible, entries, a.renderReq())
 		fileName := fmt.Sprintf("%s_page_%d", titleSlug, pageNum)
-		p, pageBytes := a.generateStoryPage(prompt, fileName, pageNum, recentRefs)
+		p, pageBytes := a.loadOrGenerate(fileName, func() (string, []byte) {
+			return a.generateStoryPage(prompt, fileName, pageNum, recentRefs)
+		})
 		if p != "" {
 			paths = append(paths, p)
 			recentRefs = appendRef(recentRefs, pageBytes)
@@ -246,8 +251,10 @@ func (a *Artist) DrawComicPages(storyText, prebuiltBible, titleSlug string, entr
 		galleryNum := i + 1
 		prompt := buildGalleryPagePrompt(style, bible, galleryNum, a.renderReq())
 		fileName := fmt.Sprintf("%s_gallery_%d", titleSlug, galleryNum)
-		gp, galleryBytes := a.generatePageWithRetry(prompt, fileName, recentRefs,
-			fmt.Sprintf("gallery page %d/%d", galleryNum, galleryPageCount))
+		gp, galleryBytes := a.loadOrGenerate(fileName, func() (string, []byte) {
+			return a.generatePageWithRetry(prompt, fileName, recentRefs,
+				fmt.Sprintf("gallery page %d/%d", galleryNum, galleryPageCount))
+		})
 		if gp != "" {
 			paths = append(paths, gp)
 			recentRefs = appendRef(recentRefs, galleryBytes)
@@ -256,7 +263,9 @@ func (a *Artist) DrawComicPages(storyText, prebuiltBible, titleSlug string, entr
 
 	// 4. Back cover — receives the same rolling refs as the last gallery page.
 	// Retried up to pageMaxRetries times; failure is non-fatal.
-	p, _ = a.generatePageWithRetry(buildBackCoverPrompt(storyText, style, bible, blurb, a.renderReq()), titleSlug+"_back", recentRefs, "back cover")
+	p, _ = a.loadOrGenerate(titleSlug+"_back", func() (string, []byte) {
+		return a.generatePageWithRetry(buildBackCoverPrompt(storyText, style, bible, blurb, a.renderReq()), titleSlug+"_back", recentRefs, "back cover")
+	})
 	if p != "" {
 		paths = append(paths, p)
 	}
@@ -309,6 +318,26 @@ func (a *Artist) generatePageWithRetry(prompt, fileName string, refs [][]byte, l
 	return "", nil
 }
 
+// loadOrGenerate returns the saved path and image bytes for fileName.
+// If the PNG already exists on disk it is loaded and returned without an API
+// call — skipping regeneration of pages that were produced in a previous run.
+// If the file is missing, generateFn is called to produce it. This lets a
+// re-run fill in only the pages that failed previously without wasting quota.
+func (a *Artist) loadOrGenerate(fileName string, generateFn func() (string, []byte)) (string, []byte) {
+	path := filepath.Join(a.outputDir, fileName+".png")
+	if _, err := os.Stat(path); err == nil {
+		// Page exists — load bytes for the reference chain and skip the API call.
+		b, readErr := os.ReadFile(path)
+		if readErr != nil {
+			fmt.Printf("  Warning: could not read existing %s for chaining: %v\n", fileName+".png", readErr)
+			return path, nil
+		}
+		fmt.Printf("  Skipping %s (already exists)\n", fileName+".png")
+		return path, b
+	}
+	return generateFn()
+}
+
 // appendRef adds imgBytes to refs and keeps at most 2 entries (cover anchor +
 // the immediately preceding page). Larger windows inflate the multimodal
 // payload significantly without proportional consistency gains.
diff --git a/internal/story/runner.go b/internal/story/runner.go
index 3299d59..c738ad0 100644
--- a/internal/story/runner.go
+++ b/internal/story/runner.go
@@ -47,6 +47,11 @@ type RunnerConfig struct {
 	// NarratorVoice picks a specific Gemini cinematic voice for narration.
 	// Empty → random pick from the curated cinematic pool each run.
 	NarratorVoice string
+	// Slug overrides the auto-generated title slug used as the output directory
+	// name and file prefix. When non-empty the runner skips slug derivation and
+	// uses this value directly. Combine with loadOrGenerate's skip-existing logic
+	// to repair a partial comic run without regenerating already-present pages.
+	Slug string
 }
 
 // Runner orchestrates the full pipeline: text → image → narration.
@@ -137,10 +142,13 @@ func (r *Runner) Run(batchFile string) error {
 		fmt.Printf("  Character bible ready from story generation (%d chars)\n", len(result.Bible))
 	}
 
-	// Derive a slug from the generated title and create the comics subfolder.
+	// Derive a slug from the generated title, or use the forced slug from config.
 	// All output files (images, PDF, story text, narration) go into comics/<slug>/.
 	slug := slugify(result.Title)
-	if result.Title != "" {
+	if r.config != nil && r.config.Slug != "" {
+		slug = r.config.Slug
+		fmt.Printf("  Comic title: %q (slug forced: %s)\n", result.Title, slug)
+	} else if result.Title != "" {
 		fmt.Printf("  Comic title: %q (slug: %s)\n", result.Title, slug)
 	}
 	comicsDir := filepath.Join(dir, "comics", slug)
diff --git a/internal/version.go b/internal/version.go
index 9f7173a..edfaf53 100644
--- a/internal/version.go
+++ b/internal/version.go
@@ -1,3 +1,3 @@
 package internal
 
-const Version = "0.17.0"
+const Version = "0.18.0"