refactor(deltaspin): encapsulate ScState lambda and Mi

[hujieting]

Summary

This PR encapsulates ScState::lambda_ and
ScState::Mi_ as private members, adds mutable
accessors, and updates all affected DeltaSpin call sites.

Motivation

The change improves data encapsulation and makes read-
only and writable access explicit. Read-only call sites
now use the existing const getters, while mutation sites
use the new mutable accessors.

Verification

• Focused DeltaSpin targets compiled successfully.
• DeltaSpin unit tests passed: 6/6.
• git diff --check passed.
• The staged agent governance check reported no errors.
• Governance warnings were limited to a migration-neutral
  global dependency count, existing test coverage without
  new test files, and no documentation update for this
  internal refactor.

Behavior

This PR does not modify the DeltaSpin numerical
algorithm, physical calculation logic, formulas, loop
behavior, or numerical parameters.