fix(stapp): claim external container before first fragment write (closes #752)

[Kailigithub]

On Streamlit >=1.60, _tick()'s first write into _stream_fh raises
"A fragment tried to write to a container created outside the fragment"
because the container position is never reserved by a full-app run. Claim
the container once on the first expander write via a single empty
st.markdown("") inside with _stream_fh:; gate subsequent writes with
a module-scope _stream_claimed flag so the claim is not re-emitted on
every 1s refresh.

Adds tests/test_stapp_fragment_container_claim.py with 4 cases that
exercise the live _tick via AST-source extraction plus a behavioural pin
of the old claim-less code.

Parent-commit proof gate:

$ cp frontends/stapp.py /tmp/stapp.py.fixed
$ git checkout main -- frontends/stapp.py
$ python3 -c "import sys; sys.path.insert(0,'tests');     import test_stapp_fragment_container_claim as m;     [getattr(m,n)() for n in dir(m) if n.startswith('test_')]"

# pre-fix result:
PASS test_pre_fix_code_would_emit_no_claim
FAIL test_tick_claims_stream_fh_exactly_once_across_multiple_ticks:
    first tick did not emit a container-claim markdown
FAIL test_tick_claims_stream_fh_on_first_expander_write:
    _tick did not emit a container-claim markdown via st.markdown("")
PASS test_tick_skips_claim_when_stream_fh_is_none
Pass: 2/4

$ cp /tmp/stapp.py.fixed frontends/stapp.py  # restore fix
$ python3 -c "..."
# post-fix result:
PASS (all 4)

The 2 failing cases on the pre-fix source are exactly the fix-detection
cases; the pre-fix pin and the skip-when-_stream_fh-is-None case pass
either way (the pin copies the OLD code body verbatim and confirms it
WOULD NOT have emitted the claim).

Fixes #752.