Checked-U32 reuse hints

H0003 previously stored bare variable names across functions and scopes, treated split as a range proof for its input, and suggested deleting a conversion whose result has a different nominal type. The editor applied that invalid edit.

The canonical contract is now reference/errors/hints.md: a hint names an existing immutable U32 binding that holds the conversion of the same unchanged Field. Tracking is deliberately restricted to straight-line lexical scopes. Entering/leaving a scope clears facts; scalar, aggregate and tuple assignment invalidate facts, as does opaque assembly. Shadowing either the input or saved result invalidates the association. Mutable result bindings are not recorded. Only the unqualified builtin as_u32 is recognized; a locally declared function of that name disables this hint, and qualified lookalikes are excluded.

split does not prove its input fits in U32. A converted Field remains nominally Field. No LSP code action removes as_u32: the diagnostic offers manual reuse of its named U32 binding instead. No standard-library conversions were removed.

Regression cases cover unrelated functions, conditional branches, loops, input mutation (including tuple assignment), mutable results, shadowed inputs/results, split, and a user-defined same-name function. A positive test checks the precise suggested binding; replacing the repeated conversion with it typechecks, while replacing it with the Field input is rejected. The editor regression requires no unsafe edit for legacy diagnostics either.

Conservative limits: facts are not joined across branches or propagated into nested scopes, even where a more complete flow analysis could safely reuse them. This favors missed optimization hints over incorrect suggestions.

Validation: cargo test --release --locked --lib passed all 674 tests, including the range-hint and LSP regressions (/tmp/h0003-full-lib.log).

Trisha release rebuild passed without Rust warnings. Re-running trisha bench passed 133/133 fixtures and 43/43 baselines, with zero H0003 diagnostics (/tmp/h0003-bench.log). This is execution validation; no proof benchmark or neural verification is implied.