feat(markdown): emit asset:N hrefs for embedded images

[alexiskowalski]

Summary

Fixes #63: embedded assets already appear as Inline::Image { source: Asset(id) } in the document model, but Markdown dropped them to alt text only (or nothing when alt was empty), so to_markdown_bytes() lost positional image markers.

Changes

• Render ImageSource::Asset as ![alt](asset:N), where N is the index into Document.assets.
• Keep ImageSource::Unavailable as alt-text-only (no bytes to reference).
• Document the asset:N scheme in the README.
• Update Markdown snapshots that previously collapsed embedded images to bare alt text.

Callers can rewrite asset:N after writing asset files out, or keep using the document model for the bytes. This is complementary to approaches that inject resolved URLs.

Test plan

• Unit tests for asset href rendering (with and without alt).
• Snapshot updates for docx/odt/rtf/doc/epub fixtures that embed pictures.
• cargo test --locked render::markdown
• cargo test --locked --test snapshots
• CI

[mirabilitum]

cargo test --locked --test snapshots fails with this PR as-is — five snapshots are missing or incorrect (ole, odp, pptx-order, rich's OLE line, and epub has the wrong content: one image + text tiny dot, not two images). This patch fixes all five; git apply from the repo root:

diff --git a/tests/snapshots/snapshots__docx__handmade-ole.docx.snap b/tests/snapshots/snapshots__docx__handmade-ole.docx.snap
index c9626e8..b9e155d 100644
--- a/tests/snapshots/snapshots__docx__handmade-ole.docx.snap
+++ b/tests/snapshots/snapshots__docx__handmade-ole.docx.snap
@@ -4,4 +4,4 @@ expression: output
 ---
 Before the object
 
-Embedded object: Excel.Sheet.12
+![Embedded object: Excel.Sheet.12](asset:0)
diff --git a/tests/snapshots/snapshots__docx__handmade-rich.docx.snap b/tests/snapshots/snapshots__docx__handmade-rich.docx.snap
index 38580ca..9114d99 100644
--- a/tests/snapshots/snapshots__docx__handmade-rich.docx.snap
+++ b/tests/snapshots/snapshots__docx__handmade-rich.docx.snap
@@ -17,6 +17,6 @@ Rich objects follow.
 
 ![tiny dot image](asset:0)
 
-Embedded object: Excel.Sheet.12
+![Embedded object: Excel.Sheet.12](asset:1)
 
 After the objects.
diff --git a/tests/snapshots/snapshots__epub__book.epub.snap b/tests/snapshots/snapshots__epub__book.epub.snap
index c0dd0de..96bbaa9 100644
--- a/tests/snapshots/snapshots__epub__book.epub.snap
+++ b/tests/snapshots/snapshots__epub__book.epub.snap
@@ -26,7 +26,7 @@ A list of things:
 
 See [Chapter Two](#epub-text-ch002-xhtml-chapter-two) for the table, or jump straight to [the marked paragraph](#epub-text-ch002-xhtml-markpoint).
 
-![tiny dot](asset:0) ![tiny dot](asset:0)
+![tiny dot](asset:0) tiny dot
 
 <a id="epub-text-ch002-xhtml-chapter-two"></a>
 
diff --git a/tests/snapshots/snapshots__odp__pres.odp.snap b/tests/snapshots/snapshots__odp__pres.odp.snap
index d46d347..e70d94b 100644
--- a/tests/snapshots/snapshots__odp__pres.odp.snap
+++ b/tests/snapshots/snapshots__odp__pres.odp.snap
@@ -17,6 +17,8 @@ Numbers Slide
 | --- | --- |
 | North | 42 |
 
+![](asset:0)
+
 Grouped shapes below.
 
 Inside a group shape.
diff --git a/tests/snapshots/snapshots__pptx__handmade-order.pptx.snap b/tests/snapshots/snapshots__pptx__handmade-order.pptx.snap
index 782ed77..652da2e 100644
--- a/tests/snapshots/snapshots__pptx__handmade-order.pptx.snap
+++ b/tests/snapshots/snapshots__pptx__handmade-order.pptx.snap
@@ -8,4 +8,4 @@ Kicker before the title
 
 Body after the title
 
-Quarterly numbers
+![Quarterly numbers](asset:0)

With it applied, cargo test --locked is green (210 unit + 8 snapshot tests).