feat: capture native structural observations

[abrichr]

What this changes

Capture now records live macOS Accessibility and Linux AT-SPI evidence beside the action event. Provider failures stay nonfatal and keep an explicit unavailable result.

The macOS provider first asks the system-wide Accessibility object for the focused element. It falls back to the frontmost application when macOS does not return that value. The Linux extra keeps PyGObject below 3.50 so it remains compatible with the documented GLib 2.64 baseline.

Stack

This PR is stacked on #94 and must merge after it. The remote #94 branch currently lacks its local frame-interval fix at fba6c745, so this draft temporarily includes that commit. The diff will narrow when #94 updates.

This change does not add the terminal manifest or sealed-load contract. That work touches #94-owned capture files and belongs in the next stack layer.

Verification

• ruff check openadapt_capture
• 587 non-slow tests passed; 2 skipped; 12 deselected
• live macOS focused-element qualification test passed
• wheel and source archive built and passed scripts/verify_distribution.py
• git diff --check passed

Two things a later reader needs

Adding an extra to pyproject.toml breaks the sealed fixtures unless uv.lock moves with it. This PR adds the linux extra. CI runs uv sync --extra dev; on main the committed lock already satisfies pyproject.toml, so nothing is rewritten and the fixtures' pinned builder.lock_sha256 still matches. With a new extra the lock goes stale, uv sync re-locks, sha256(uv.lock) changes, and tests/test_synthetic_fixtures.py fails on both fixtures for every Python version. Measured in one worktree with one uv: main's pair produced a stable lock fb9a77ab..., this branch's pyproject.toml produced a rewritten 8758039f.... #107 later narrowed the recorded provenance to the determinants that actually decide the fixture bytes, which is what resolved this. The narrowing was established by experiment: re-locking and regenerating with the old hash held produced byte-identical artifacts, so the lock's contents were causally inert and only its recorded hash was not.

A pre-rebase commit is preserved at rescue/capture-96-pre-forcepush-c61d17a. Two files live only there and are referenced by nothing on main or on this head: scripts/read_structural_fixture_output.py and its test tests/test_structural_fixture_output.py. They look like a fixture-output reader someone wrote and then lost. Take them from the rescue branch if you want them.