tests: a whole-output capture must not land in the work tree

[tannevaled]

The defect

Three live tests photograph the entire output with grim <file> — no
geometry — and then copied the result next to the source, under a name nothing
ignores:

file	capture
damage_integration_test.go	live-wayland-damage.png
wayland_integration_test.go	live-wayland-capture.png
wlhidpi_integration_test.go	live-hidpi-<name>.png

Under headless sway in CI the output holds only the test window. In a person's
own session it holds their screen — their code, their mail, their tabs —
and this repository is public. A .gitignore entry would be a safety net
rather than a barrier: git add -f, a fresh clone, or any tool that does not
consult it publishes the file anyway.

They now go through captureDir: somewhere durable, and inside no git work
tree.

One rule, one place

internal/cocoa already had that rule, for exactly this reason (#68), and it
was one package's private copy. It moves to internal/capture, and both
packages read it there.

What is deliberately left alone

The import -window <id> captures stay where they are. They photograph the
test's own window and nothing else, and this project keeps dated proof
images as reviewable evidence on purpose — the .gitignore says so. Routing
them would have been tidiness overriding a decision somebody made.

Tests

The guard that matters is witnessed: remove the repository check and
TestDirRefusesARepository says so by name. Both the override path and the
default path are covered, as are the two failures a run can really meet (no
configuration directory; a destination under a regular file).

internal/capture is covered but for one branch: filepath.Abs fails only when
the working directory cannot be read, which an ordinary run never meets. It is
kept and its unreachability is stated where it sits — answering a half-resolved
path is exactly how a capture escapes its directory.

🤖 Generated with Claude Code