fix(dfir_lang): order loop-ingress senders before the whole loop block, fix #3048

[MingweiSamuel]

make_loops_contiguous gathers every subgraph of a loop to the flat-order
position of the loop's first subgraph. This is only sound if every subgraph
feeding the loop from outside (loop ingress) is already ordered before the
loop's earliest subgraph. Otherwise gathering hoists an ingress receiver
inside the loop ahead of its external sender, violating topological order.
Downstream this produced a handoff-buffer "use before declaration" compile
error (and, logically, a drained-but-never-filled batch).

Fix: in find_subgraph_unionfind, add loop-ingress ordering constraints to the
predecessor set that feeds the (existing, unified) topological sort. For each
forward (non-tick) edge src -> dst where dst lies inside a loop that does
not contain src, require src to precede every node inside the outermost
such loop — i.e. the whole top-level loop block that will be gathered. Because
this reuses the same toposort, all other ordering constraints (handoff
references, access groups) are respected automatically, and non-loop programs
are entirely unaffected (no constraints are added). A genuine
loop -> external -> loop dependency (which cannot be made contiguous) now
surfaces as the usual intra-tick cycle error.

Also adds a defensive assertion after make_loops_contiguous that verifies the
toposort is still valid.

Tests:

• dfir_lang: test_make_loops_contiguous_ingress_toposort (previously
  #[ignore]d as a repro) now asserts a valid toposort and passes.
• dfir_rs surface_loop: adds test_loop_ingress_forward_ref_sources, an
  end-to-end surface-syntax repro (two ingress points with sources declared
  after the loop) that both compiles and delivers all data. Both tests were
  confirmed to fail without the fix (the integration test fails at macro
  expansion via the new debug assertion).

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Pull request overview

This PR fixes a DFIR graph ordering bug where make_loops_contiguous could hoist loop-ingress receivers ahead of their external senders, breaking topological order (issue #3048). It does so by adding loop-ingress ordering constraints into the existing predecessor set that feeds the unified toposort, and adds regression coverage plus a defensive post-condition check.

Changes:

• Add loop-ingress ordering constraints during subgraph union-find/toposort construction to keep gathered loop blocks topologically valid.
• Add a defensive post-make_loops_contiguous topo validation check (plus a reusable validate_topo_sort helper).
• Add regression tests in both dfir_lang (unit) and dfir_rs (end-to-end surface syntax).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
dfir_rs/tests/surface_loop.rs	Adds an end-to-end regression test covering forward-referenced loop ingress sources.
dfir_lang/src/graph/meta_graph.rs	Exposes loop member nodes via a new loop_nodes(loop_id) accessor.
dfir_lang/src/graph/graph_algorithms.rs	Adds validate_topo_sort utility and unit tests for it.
dfir_lang/src/graph/flat_to_partitioned.rs	Adds loop-ingress ordering constraints and a post-contiguity topo validation check; includes regression test.

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