perf(coverage): the DEBUG trap re-mangles the file path twice per executed line #1102

Problem

With the report phase down to ~1s (#1088, #1090) and the buffer gone (#1101),
capture is what a --coverage run costs. record_line runs per executed line
and does two cache lookups — the tracked decision and the normalized path — and
each one rebuilds its key with

_BASHUNIT_COVERAGE_LOOKUP_KEY_OUT="$1${2//[^a-zA-Z0-9]/_}"

That pattern substitution is the whole cost, and it is not linear in path
length. Measured on Bash 3.2 arm64, 5000 mangles:

So record_line costs 402 µs per line for a real path
(/Users/…/src/coverage/engine.sh), of which 328 µs is the two lookups and
only 29 µs the actual append. A project whose sources sit deeper pays
quadratically more, for nothing it can control.

Proposal

A one-entry memo for the last file seen. Consecutive executed lines come from
the same file almost always — that is what a function body is — so a single
string compare replaces both key rebuilds and both indirect reads:

Expected: ~402 µs to ~75 µs per line, and the path-length sensitivity mostly
gone. The existing per-path caches stay for the miss path.

coverage::init has to clear the memo along with the other in-memory caches,
or a second run in the same shell inherits a stale decision.