fix(hir-def): don't panic instantiating cross-file extends chains (#236)

[avrabe]

Fixes #236 — spar instance panicked with index out of bounds: the len is 0 but the index is 0 (instance.rs) when a system implementation extends a parent declared in a different file.

Cause

collect_impl_chain walks the extends chain across trees (files) and collects each inherited subcomponent's SubcomponentIdx — but dropped the tree it was declared in. The caller then paired every inherited sub_idx with the most-derived impl's loc.tree. When that derived impl declares no subcomponents of its own, its tree's subcomponents slice is empty, so subcomponents[sub_idx] (index 0 into len 0) panicked.

Fix

Carry the originating tree_idx per subcomponent through ImplChainResult.subcomponents, and use it instead of loc.tree at the three consumer sites: recursive instantiation (Some((sub_tree, sub_idx))), build_leaf_property_map, and build_anon_property_map.

Verification

• Reproduced with the reporter's files (dataflow.aadl + dataflow_diagram.aadl + hw.aadl): panic before, full instance tree after.
• New regression test extends_chain_across_files_does_not_panic — base impl with a subcomponent + a derived impl that extends it from a separate tree with no own subcomponents (the exact panic shape); asserts the inherited subcomponent instantiates.
• cargo test -p spar-hir-def: 458 + 1 pass, 0 failures.

Note

The reporter's model also surfaces applies_to path 'my_process.c1' could not be resolved warnings for connection properties — that's the separate #237 (properties on connections/features), not addressed here. This PR is scoped to the panic.

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_{Updated automatically by tools/post_verification_comment.py. Source of truth: artifacts/verification.yaml.}

[codecov]

Codecov Report

❌ Patch coverage is 94.73684% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/spar-hir-def/src/instance.rs	94.73%	3 Missing ⚠️

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