emit ItemType::Image bboxes for Image XObjects (was: silently dropped) (#94)

* emit ItemType::Image bboxes for Image XObjects (was: silently dropped)

Background. ItemType::Image, MarkdownOptions::include_images, and the
markdown emitter's image-collection path have all been in the tree
for a while, but no producer ever populated them — content_stream.rs
explicitly `// Skip images — text extraction only` at the Do
operator, and the nested Form-XObject walker in xobjects.rs only
matched XObjectType::Form, silently dropping Image entries. The
declared types were dead code.

This PR lights them up. At every Do that resolves to an Image
XObject (both top-level and nested inside Form XObjects), we now
compute the page-space bbox from the current CTM via a new
`image_bbox_from_ctm` helper — handling both axis-aligned and
rotated/sheared placements via 4-corner AABB — and emit a TextItem
with `item_type: ItemType::Image` and the legacy `[Image: <name>]`
text payload that the markdown emitter already knows how to render.

Callers can now find raster figures via `extract_text_with_positions`
(and the `_mem` variant, newly re-exported at the crate root) without
needing to re-parse the PDF or run a vision/layout model. The intended
consumer is layout-aware text pipelines that want to crop figures and
caption them out-of-band.

Two backstops to avoid silent breakage for existing callers:

  1. `MarkdownOptions::include_images` default flipped `true → false`.
     If it stayed at `true`, every existing user of
     `extract_pages_markdown` would suddenly see `![Image: Im0](image)`
     placeholders inserted throughout their output the moment they
     upgraded. Image data is still available structurally via
     `extract_text_with_positions`; rendering it into markdown is now
     an opt-in. New regression test asserts `extract_pages_markdown`
     output is unchanged for the image-bearing fixture.

  2. Image items now also skip the layout heuristics
     (`detect_columns`, `detect_tables_from_rects`) via a new
     `is_text_layout_item` predicate. Without this filter, an image's
     left edge would land in the column-projection profile and skew
     table column detection — surfaced by
     `vector_grid_tests::upstage_key_functions_four_cols` going from 4
     detected columns to 5 in CI before the filter was added.

Re-exporting `extract_text_with_positions_mem` at the crate root —
strictly additive; mirrors how `extract_pages_markdown_mem` is already
available there.

Tests:

  - test_extract_text_with_positions_emits_image_bboxes — minimal PDF
    with one 200×100 image at (50, 600); asserts one Image item with
    correct bbox + page + text.
  - test_image_xobject_bbox_handles_rotated_ctm — 90° rotated image
    via shear-component CTM; asserts AABB is correct (handles non-
    axis-aligned placements via 4-corner clamp).
  - test_image_emission_does_not_change_default_markdown — asserts no
    `Image:` token leaks into default markdown output, regression
    guard for the include_images flip.
  - test_markdown_options_default_has_include_images_false — explicit
    sentinel so anyone flipping it back catches it in CI.

* Bump version from 1.8.15 to 1.9.0

napi/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@firecrawl/pdf-inspector",
-  "version": "1.8.15",
+  "version": "1.9.0",
   "description": "Fast PDF classification and text extraction. Detect text-based vs scanned PDFs, extract text by region with quality checks. Native Rust performance via napi-rs.",
   "main": "index.js",
   "types": "index.d.ts",