feat(migration): tentative-binding overlay (Track E commit 2/8)

[avrabe]

Summary

• Adds BindingOverlay in spar-hir-def — an immutable per-component-instance map from a moving component to its hypothetical target processor — so any analysis can run "as if" the binding were rebound without mutating the underlying SystemInstance.
• Adds actual_processor_binding_with_overlay(instance, comp, overlay) as the load-bearing accessor: when the overlay carries a move it returns the hypothetical target; otherwise it falls through to the declared Actual_Processor_Binding.
• Validates the overlay against Spar_Migration::Frozen (from commit 1) and Spar_Migration::Allowed_Targets, returning structured FrozenViolation / AllowedTargetsViolation diagnostics (no prose-parsing required by callers).
• Empty overlay is a strict no-op identity — actual_processor_binding_with_overlay(.., None) and actual_processor_binding_with_overlay(.., Some(&BindingOverlay::new())) agree on every component.
• Per Track E research §6.4 (design doc).

Scope (what this commit is)

• crates/spar-hir-def/src/overlay.rs — BindingOverlay, FrozenViolation, AllowedTargetsViolation, OverlayDiagnostic, actual_processor_binding_with_overlay, plus 10 unit tests.
• Public re-exports added to crates/spar-hir-def/src/lib.rs.
• REQ-MIGRATION-004 appended to artifacts/requirements.yaml.
• TEST-OVERLAY appended to artifacts/verification.yaml, satisfying REQ-MIGRATION-{004,001,002}.

Out of scope (explicitly NOT in this commit)

• CLI surface. spar moves verify lands in commit 3.
• Solver integration. spar-solver hypothetical mode lands in commits 4–5.
• MCP surface. spar.verify_move, spar.enumerate_moves are deferred to v0.9.0 (commits 9–11).
• Memory / connection bindings. Processor binding only for v0.8.0 — memory and connection bindings may join the overlay in v0.9.0.
• Variant resolution. rivet variant composition happens before the overlay, not inside it (docs: rivet <-> spar variant binding contract v1 (proposed) #144 wiring lands in commit 7).
• No changes to COMPLIANCE.md, spar-cli, spar-solver, α/β/δ surfaces, or any MCP scaffolding.

Test plan

• cargo build --workspace — clean
• cargo test -p spar-hir-def — 448 passed (438 prior + 10 new in overlay::tests)
• cargo test --workspace — all green
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo fmt --all -- --check — clean
• rivet validate — PASS

The 10 new tests cover the contract: empty-overlay identity, single-move semantics, unrelated-component non-interference, frozen-component violation (with Pinned_Reason plumbed through), non-frozen no-op, allowed-targets satisfied, allowed-targets violated, empty-allowed-targets means no restriction (both empty list and absent property), three-move independence, and clone independence.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 95.56962% with 21 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/spar-hir-def/src/overlay.rs	95.56%	21 Missing ⚠️

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