Preserve lexical occurrence roles in let initializers

[briansrls]

Summary

Make ExprLet initializer children classify from their own syntax instead of inheriting the generic declaration-head TypeOccurrence role. The explicit type-annotation edge remains TypeOccurrence.

Add a modeled parser witness for let x = y that requires the x declaration and y initializer reference to remain LexicalValueOccurrence while both Int annotations remain TypeOccurrence. Expand the existing generated production serde witness to compile two modules with equal authored names, prove graph-scoped occurrence IDs are disjoint, and round-trip each TypedModule plus the enclosing ResolvedGraph exactly.

This is a bounded follow-up to #7352. It does not add resolver selection, Node-attached identity, ownership, emitter/runtime behavior, or premature stage0 adoption of std.occurrence_binding.

Test plan

• target/release/claim_batch ... --functions w_pattern_parser_materializes_dedicated_declaration_nodes,...,w_production_frontend_refuses_duplicate_reference_rows — 13/13 PASS.
• cargo test --manifest-path src/v1/stage0/Cargo.toml pipeline_occurrence_sidecar_serde_round_trip -- --nocapture — PASS; two production modules, standalone TypedModule serde, ResolvedGraph serde, and disjoint IDs.
• target/release/regen_stage0 --verify — regen_divergence_count=0.
• cargo fmt --all -- --check — PASS.
• git diff --check — PASS.

Original parent: 220b556d537e9cce55f419c3ebfbac9c5eb80f72 (#7352).
Current-main synchronization: merge commit 69a44bbd2da82c0b031008c9c2af8107ca9b7c7a includes origin/main a00048fd47; no occurrence-transport conflicts or product edits.