check-mix.py resolved the live session from argv only, so any Ardour
launched from the GUI's recent-sessions list (bare argv) went unresolved
and check-mix silently fell back to a guessed path. gig-log.py's FilesLens
already solved this with a 3-rung evidence ladder (argv > cwd > open fds,
commit abad7515) and had a sync comment flagging check-mix as needing it.

Extracted the ladder into tools/ardour_session.py — one resolver instead
of two, per feedback_one_parser_per_concept — and made both gig-log.py and
check-mix.py import it. gig-log's FilesLens keeps its instance-level
caching/logging but delegates all evidence-gathering to the shared module;
the now-unnecessary "keep in sync" comment is gone.

Verified: gig-log's 169 pytest tests still pass unchanged (they exercise
all 3 rungs against synthetic /proc fixtures). Live: Ardour was running
("Tidal Live" session, launched with argv), so check-mix.py resolved it
for real and reported per-orbit gains — surfacing the same recurring bug
this fix was meant to catch: Tidal 07/08/10/12 sit at -inf dB in the
currently SAVED session. Rungs 2 (cwd) and 3 (open-fds) were proven against
synthetic fixtures matching the live-rig gap, since the running process's
argv already carried the session path.