Align checksums with bDS2.

specs/import.allium

@@ -264,6 +264,8 @@ invariant WxrConversionParity {
 }
 
 invariant AnalysisClassification {
+    -- Post/page checksums are lower-hex SHA-256 of converted Markdown; media
+    -- checksums are lower-hex MD5 of source bytes, matching bDS2.
     -- Posts/pages: same slug and checksum => update; same slug with different
     -- content => conflict; checksum matching another post => content duplicate;
     -- otherwise new. Media applies the same rules by original filename and

specs/media_processing.allium

@@ -254,7 +254,8 @@ invariant MediaTranslationFileLayout {
 rule ImportMedia {
     when: ImportMediaRequested(source_path, project, metadata)
     -- metadata is optional import context: title, alt, caption, author,
-    -- language, and tags may be supplied by the caller.
+    -- language, tags, and a checksum may be supplied by the caller. WXR
+    -- supplies a lower-hex MD5 checksum; interactive imports leave it nil.
     -- 1. Validate file type (must be supported image)
     -- 2. Generate UUID v4 filename
     -- 3. Copy to media/{YYYY}/{MM}/{uuid}.{ext}
@@ -276,7 +277,7 @@ rule ImportMedia {
         tags: metadata.tags,
         file_path: format("media/{yyyy}/{mm}/{uuid}.{ext}"),
         sidecar_path: format("media/{yyyy}/{mm}/{uuid}.{ext}.meta"),
-        checksum: sha256(source_path)
+        checksum: metadata.checksum
     )
     ensures: ThumbnailsGenerated(media_id)
     ensures: SearchIndexUpdated(media_id)

specs/post.allium

@@ -98,6 +98,8 @@ entity Post {
     template_slug: String?
     file_path: String
     checksum: String?
+        -- Caller-supplied SHA-256 content change signal. Publish preserves it;
+        -- filesystem rebuild/sync clears it to match bDS2.
     tags: List<String>
     categories: List<String>
     created_at: Timestamp
@@ -210,6 +212,7 @@ rule PublishPost {
         -- Writes frontmatter + markdown to posts/YYYY/MM/{slug}.md
     ensures: post.content = null
         -- Content cleared from DB; now lives in filesystem only
+    -- post.checksum is preserved unchanged.
     ensures: SearchIndexUpdated(post)
     ensures: PostLinksUpdated(post)
         -- Parse inter-post links, update link graph
@@ -242,6 +245,7 @@ rule SyncPostFromFile {
     -- record from it (the filesystem is treated as truth for that one post).
     -- Re-syncs the post's link graph. Errors out if the file is missing.
     ensures: PostFieldsUpdated(post, parse_post_file(post.file_path))
+    ensures: post.checksum = null
     ensures: PostLinksUpdated(post)
 }
 

specs/schema.allium

@@ -55,7 +55,7 @@ entity Post {
     updated_at: Timestamp
     published_at: Timestamp?
     file_path: String       -- Empty for never-published drafts
-    checksum: String?       -- SHA-256 of content
+    checksum: String?       -- Caller-supplied SHA-256 content change signal; nil after filesystem rebuild
     tags: Set<String>       -- JSON array stored as text
     categories: Set<String> -- JSON array stored as text
     template_slug: String?  -- User template override
@@ -83,7 +83,7 @@ entity PostTranslation {
     updated_at: Timestamp
     published_at: Timestamp?
     file_path: String
-    checksum: String?
+    checksum: String?       -- Caller-supplied content change signal; nil after filesystem rebuild
 }
 
 entity Media {
@@ -103,7 +103,7 @@ entity Media {
     sidecar_path: String    -- Path to .meta sidecar file
     created_at: Timestamp
     updated_at: Timestamp
-    checksum: String?
+    checksum: String?       -- Lower-hex MD5 of media bytes when known
     tags: Set<String>       -- JSON array stored as text
     language: String?       -- ISO 639-1 code
 }

specs/translation.allium

@@ -51,6 +51,8 @@ entity PostTranslation {
     status: PostTranslationStatus
     file_path: String
     checksum: String?
+        -- Caller-supplied content change signal. Publish preserves it;
+        -- filesystem rebuild/sync clears it to match bDS2.
     created_at: Timestamp
     updated_at: Timestamp
     published_at: Timestamp?
@@ -143,6 +145,7 @@ rule PublishTranslation {
     ensures: TranslationFileWritten(translation)
     ensures: translation.content = null
         -- Content moves to filesystem
+    -- translation.checksum is preserved unchanged.
 }
 
 rule ReopenPublishedTranslation {