[mono-move] Implement integer cast micro-op

[vgao1996]

Summary

Implements the family of integer cast micro-ops for the mono-move VM as a single universal IntCast micro-op that covers all 12×12 (from, to) integer type pairs.

The cast is value-preserving: it reproduces the source's mathematical integer value in the target type and aborts iff that value falls outside the target's [MIN, MAX] range. This matches the existing Move VM's Value::cast_* semantics exactly (widening, narrowing, and signed↔unsigned).

Self-casts (from == to) are accepted and run as identity — the bytecode verifier admits them (the operand only has to be an integer) and the production VM executes them, so mono-move must not reject them.

Changes

• core: IntCastOp operand + MicroOp::IntCast, with Display, gas cost, is_allocating, and re-exports.
• runtime: exec_int_cast doing a uniform per-pair <dst>::try_from(src) dispatch (native↔native via std, native↔256-bit via the int256 crate, identity via the blanket impl); a new CastOutOfRange error; verifier frame-access checks.
• specializer: lowers UnaryOp::Cast* to IntCast, with the target type resolved by a new UnaryOp::extract_to_int_ty helper.

Testing

• Property tests (runtime/tests/arithmetic_ops.rs): all 144 (from, to) pairs, each checked against an independent num-bigint range-check oracle — one generated test per pair.
• Differential tests: int_cast.move covers every cast category (widen / narrow / signed↔unsigned, both success and out-of-range) for the wide integer types end-to-end against the legacy VM; self_cast.masm pins the self-cast identity behavior.

Note: the narrow widths (u8..u32, i8..i32) can't yet flow through the .move pipeline (the 8-byte slot-align rule), so they're covered by the property tests rather than the differential .move tests.

🤖 Generated with Claude Code

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Note

Medium Risk
Touches VM execution semantics for all integer cast combinations; risk is mitigated by extensive property and differential tests, but incorrect range handling could diverge from the legacy VM.

Overview
Adds a universal MicroOp::IntCast path so mono-move can execute Move integer casts end-to-end: IntCastOp in core (display, gas, non-allocating), exec_int_cast in the runtime via nested try_from over all 12×12 type pairs with CastOutOfRange, verifier frame checks (including identity casts), and specializer lowering of UnaryOp::Cast* through UnaryOp::extract_to_int_ty.

Semantics are value-preserving in the target range (aligned with the legacy VM); out-of-range values abort. Coverage includes 144 proptest pairs in arithmetic_ops.rs plus differential int_cast / self_cast testsuite cases against the production VM.

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[vgao1996]

Note: my plan is to rename this file to int_ops.rs before landing, but keeping the name now so it doesn't screw up the review.