fix(cua-driver): report Windows tree completeness

[ethanblake4]

On Windows, elements_complete is currently meaningless; it is unconditionally assigned false because the walker does not track whether it hit max_elements or max_depth.

This PR tracks whether the Windows UIA walk actually skips nodes because of max_elements, max_depth, cache-request setup, or enumeration failures, and reports elements_complete from that traversal rather than always false. Query projection does not change underlying completeness; partial MSAA and unusable trees remain conservative.

The follow-up removes the dead walk_cached compatibility shim, historical bound aliases, and an unreachable second SAL fallback branch. SAL windows already return through the earlier MSAA path.

Validation:

• cargo fmt --all -- --check
• git diff --check
• contributor validation: cargo test -p platform-windows uia::tests --locked; cargo check -p cua-driver --locked; schema consistency; WPF harness query/completeness row
• pending on current head: Windows compile/unit CI and canonical WPF harness replay

[injaneity]

looks good! just a few changes needed:

• remove the unused walk_cached compatibility shim and its historical max_* aliases instead of adapting dead code.
• combine the four synthetic tests into a table-driven test. the query unit test is especially weak because it tests filtering and completeness independently; the wpf test already verifies their integration. the naming is not really representative of what the tests do

[ethanblake4]

thanks @injaneity ! Addressed those issues, does the new test setup look ok?