Record the paths of the content model as columns in the engine #615

Context

DESIGN.md section 10.2: "Characters and content items are columnar buffers that NumPy and Arrow read without copying; records are views over them." #606 gave the paths of a page columns, Page.paths(), in the layout agreed there, and built them in the binding from the interpretation that Page.content() memoizes (section 5), as its Paths asked. That interpretation keeps one tuple per path in the engine's content model, ContentItem::Path: 288 bytes of enum and three heap allocations for the verbs, the coordinates, and the stroke's paint. The page memo holds those tuples after the call, and building them dominates what reading a CAD page costs once the Python objects are gone.

On the 200,000-line page of #606, on Windows 11 x86-64 (evidence/path-columns/results/windows-amd64.md):

• Page.paths() takes 0.25 s, a little longer than rendering the page (0.21 s), against 4.0 s for Content.paths. Once the interpretation is memoized, building the columns takes 36 ms of that.
• The columns hold 224 bytes a path: 118 for the path and 53 for each of its two segments.
• The call grows the peak resident set by 581 bytes a path and leaves 693 held. About 470 of those 693 are the memoized tuples.

Goal

The content collector records paths as columns in the engine, the page memo holds them, and Page.paths() hands the memo's buffers to Python without a copy, so that reading the paths of a CAD page costs tens of bytes per segment in all, not only in the columns that Python holds.

Scope

In:

• crates/pyropdf-engine/src/extract/model.rs: a columnar path table that the collector appends to, with the fill-and-stroke merge and the clip filter of today's records, and a PageContent whose items refer to its rows.
• crates/pyropdf-python/src/buffers.rs and items.rs: PathColumns over the memoized table, and Page.content()'s path tuples built from it.

Out:

• The public layout of Paths, agreed in Read the paths of a page as columnar buffers #606.
• Text runs and images.
• Making the Content.paths records views over the columns.

Paths

crates/pyropdf-engine/src/extract/model.rs, crates/pyropdf-engine/src/extract/mod.rs, crates/pyropdf-engine/src/extract/tests.rs, crates/pyropdf-python/src/buffers.rs, crates/pyropdf-python/src/items.rs, and evidence/path-columns/.

Acceptance

• tests/test_path_columns.py and tests/test_content_model.py pass unchanged, and max_elements refuses with content_element_count at the same bounds
• evidence/path-columns/measure.py rerun shows columns costing at most a few hundred bytes of peak and held memory per line on the 200,000-line page, with the results committed

Notes

• The collector merges a stroke into the fill before it when both paint the same segments (record_path), and finish drops the items outside clip after the budget has counted them; the table needs both.