A @data_provider test shares one snapshot across all its values

[Chemaclass]

Problem

An unnamed snapshot is named after the test function, and a @data_provider
runs that function once per value — so every value compares against the same
file:

function provide_values() { echo "alpha"; echo "beta"; }

# @data_provider provide_values
function test_shared() { assert_match_snapshot "value is $1"; }

The first value creates the snapshot; the second fails with
Expected to match the snapshot even though nothing about it is wrong. The
message is accurate about the symptom and silent about the cause.

What I changed, and what I did not

Documented the interaction in docs/assertions.md, pointing at
assert_match_named_snapshot "$1" for one file per value.

I tried putting the hint in the failure message and reverted it: the
renderer has no signal for "this test ran with provider arguments".
_BASHUNIT_CURRENT_TEST_INTERPOLATED_NAME looked like one but is only set for
::N::-style interpolated titles, so the hint never fired. Exposing such a
signal is a design change rather than a message fix, and it is the maintainer's
call.

Giving each value its own snapshot file would be the other fix, and a bigger
one: it would orphan every snapshot already on disk.

An acceptance test now pins the current behaviour either way.

[Chemaclass]

Done in #1186 (merged).