fix(svelte): stamp dynamic-import stub source_file to target, not importer (#712)

[jippi]

Code written by Claude Code, but reviewed by me (I'm not super familiar with Python, but SWE with +20 years experience).

Fixes #712. Independent of #714 — branched off v7, applies cleanly with or without #714 merged first.

Problem

extract_svelte()'s regex pass for import('...') resolved a target path (relative or via tsconfig alias) and created a stub node for it — but stamped the stub's source_file to str(path), which is the importer's path, not the target's.

build_from_json does last-write-wins on node attributes (G.add_node overwrites). When the target file is later extracted by _extract_generic on its own, both nodes share the same id (_make_id(str(target))) and merge. Whether the stub's wrong source_file or the real correct one wins depends purely on file-iteration order — non-deterministic.

Effect: downstream tools that read source_file off file nodes (display, "where is X defined", blast-radius queries, community summaries) see the file as living inside whichever component first imported it. On a real 1,873-file SvelteKit codebase, 16 .svelte file nodes were corrupted this way — every dynamically-imported component sampled.

Fix

Thread the resolved target path through to source_file:

• relative imports → str(resolved)
• alias imports ($lib/...) → str(resolved_alias)
• bare/scoped externals → leave source_file empty (we genuinely don't know where node_modules live, and stamping the importer would lie)

The stub edge's source_file is unchanged — that legitimately is the importer (where the import statement lives). Edge vs node distinction is preserved.

Tests

7 new tests in tests/test_svelte_dynamic_import_stub.py:

Test	What it guards
test_relative_dynamic_import_stub_points_to_target	Stub source_file == target's path
test_relative_dynamic_import_stub_does_not_carry_importer_path	The literal #712 bug
test_alias_dynamic_import_stub_points_to_resolved_path	$lib/... aliases resolve correctly
test_two_importers_agree_on_target_source_file	Two stubs of same target are consistent → merge-order independent
test_target_extraction_does_not_clobber_correct_source_file	End-to-end: stub's source_file matches real extraction's source_file
test_bare_module_dynamic_import_does_not_corrupt_source_file	External modules don't get importer-path stamped
test_dynamic_import_edge_source_file_is_importer	Edge source_file unchanged (over-correction guard)

6 of 7 fail against unpatched v7 — confirming they're real regression guards. The 7th is the over-correction check that intentionally documents what should NOT change (edge source_file remains the importer).

Test plan

• pytest tests/test_svelte_dynamic_import_stub.py — 7/7 pass
• Confirmed 6/7 fail against unpatched v7
• pytest tests/ — 556 pass, 7 pre-existing failures unrelated to this change