refactor(dfir_lang): more precise span on generic diagnostics

[MingweiSamuel]

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[Copilot]

Pull request overview

Refines dfir_lang operator-generic diagnostics so errors highlight the specific lifetime vs type-argument portion of the generic argument list, and updates compile-fail expectations accordingly.

Changes:

• Added OpInstGenerics::{persistence_args_span,type_args_span} helpers to compute tighter spans for diagnostics.
• Updated generic-argument constraint diagnostics to use the more precise spans when available.
• Updated compile-fail .stderr fixtures for lattice operator wrong-generic tests to match the new caret locations.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
dfir_lang/src/graph/mod.rs	Adds span-derivation helpers for persistence vs type generic arguments.
dfir_lang/src/graph/meta_graph.rs	Uses the new helpers to emit more precise Diagnostic::spanned locations.
dfir_rs/tests/compile-fail-stable/surface_lattice_reduce_wronggeneric.stderr	Updates expected diagnostic span/caret location.
dfir_rs/tests/compile-fail-stable/surface_lattice_fold_wronggeneric.stderr	Updates expected diagnostic span/caret location.

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