feat(hir-def): retain component EMV2 subclause in the item-tree (REQ-EMV2-PROPAGATION-002, L2/4)

[avrabe]

What

Layer 2 of 4 of the EMV2 annex→instance foundation (#294). A component's annex EMV2 {** … **} subclause was dropped at item-tree lowering (lower_component_type/impl had no ANNEX_SUBCLAUSE handling — the empty arm at lower.rs:169 is the package-section one). This retains it:

• New Emv2Subclause { name, model: Emv2Model } stored in an emv2_subclauses arena on ItemTree, referenced from ComponentTypeItem.emv2 / ComponentImplItem.emv2 (Vec<Idx> — the idiomatic nested-item slot; the owning classifier is the item, so L3 looks it up by name).
• lower_emv2_subclauses walks the component's ANNEX_SUBCLAUSE children (they repeat per AS-5506B → filter, not find), recovers the body via the existing spar_annex::extract_annex_content, and reuses L1 (emv2::parse → Emv2Model::from_syntax). Non-EMV2 annex languages stay dropped (case-insensitive name filter).
• spar-hir-def gains a spar-annex dep — already beneath it via spar-base-db, so acyclic; Emv2Model's Debug/Clone/PartialEq/Eq matches the item structs' derives, so no mirror types and no salsa Hash/Update needed.

Scope

Stops at the item-tree/type level. Does not touch instantiation — the diff to instance.rs/resolver.rs is purely mechanical emv2: Vec::new() additions to hand-built test fixtures. Projecting the retained model onto ComponentInstances is Layer 3; building the Emv2Overlay + wiring spar analyze (the end-to-end ">0 chains" oracle) is Layer 4.

Verification

• Oracle-first, non-circular (3 tests): parse real AADL → item-tree → assert the lowered model equals L1 parsing the same snippet directly (never a hand-built struct). (a) a system TYPE retains one subclause whose model equals the oracle and has exactly 2 propagations + 3 flows (non-vacuous); (b) a system IMPLEMENTATION retains it on its own list; (c) EMV2 + a non-EMV2 annex retains exactly the EMV2 one (guards the filter). Uses the verbatim OSATE-corpus block L1 pins.
• cargo test -p spar-hir-def → 464 + 3 pass; clippy + fmt clean.
• Mutation on lower_emv2_subclauses: 0 missed (3 caught, 1 unviable).
• Clean-room verify: GO — a cold agent confirmed all 6 claims (retention in both paths, filter-not-find, non-EMV2 dropped, non-circular + non-vacuous oracle, no instantiation overreach, acyclic dep), and mutation-tested the filter to prove the test is non-vacuous.
• Traceability: TEST-EMV2-ITEMTREE-ATTACH verifies REQ-EMV2-PROPAGATION-002 (umbrella stays proposed until L4's end-to-end oracle).

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[github-actions]

Rivet verification gate

✅ 20/20 passed

	count
Passed	20
Failed	0
Skipped (no steps)	0

Filter: (and (= type "feature") (or (has-tag "v093") (has-tag "v0100")))

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

[codecov]

Codecov Report

❌ Patch coverage is 99.40828% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/spar-hir-def/src/item_tree/lower.rs	95.65%	1 Missing ⚠️

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