Handle inline (non-reference) structure elements

[gaoflow]

The bug

PDF.structure_tree / Page.structure_tree raise KeyError when a structure element is embedded inline — as a direct dictionary in another element's /K — rather than as an indirect reference:

File ".../pdfplumber/structure.py", line 407, in prune
    children = prune(children)
File ".../pdfplumber/structure.py", line 406, in prune
    element, children = s[repr(ref)]
KeyError: "{'Type': /'StructElem', 'S': /'P', 'Pg': <PDFObjRef:3>, 'K': 0}"

Per PDF 1.7 §14.7.2, the /K value "may be ... a structure element dictionary ... or an array of such". _make_element already has an explicit branch for an inline-dict child (# a single object.. ugh...), and _parse_struct_tree / _resolve_children already handle an inline dict at the root — but the per-child traversal does not, so a nested inline element crashes.

Root cause

In _parse_struct_tree's main loop, only PDFObjRef children are pushed onto the work queue (and thus registered in the s lookup). An inline structure-element dict is silently skipped, so it never gets an s entry — and prune() then hits s[repr(ref)] → KeyError. _resolve_children follows the same PDFObjRef-only rule, so even past the crash an inline child would be dropped from the output.

The fix

Two symmetric one-line changes: in both the traversal loop and _resolve_children, treat a child that is a structure-element dictionary the same as an indirect reference (isinstance(child, (PDFObjRef, dict))). MCID dicts were already handled/continued earlier and are not in the lookup, so they're unaffected; prune() needs no change once inline elements are registered.

Tests

test_inline_structure_element builds a minimal tagged PDF in memory whose Document element holds an inline P element in its /K array, and asserts the parsed tree on both the document-level (pdf.structure_tree) and page-level (page.structure_tree) code paths. It fails on develop with the exact KeyError and passes with the fix; tests/test_structure.py + tests/test_mcids.py + tests/test_basics.py stay green (39 passed); black/isort/flake8 clean. CHANGELOG updated.

(None of the 65 shipped test PDFs exercise inline structure children, so this path was previously untested.)