Clarify NAV semantics: make navBalance optional in FYV

[liobrasil]

Summary

• make Strategy.navBalance return UFix64? and default to nil instead of falling back to availableBalance
• make YieldVault.getNAVBalance return UFix64?
• update PMStrategiesV1 NAV overrides/helpers to return optional values
• in _navBalanceFor, return nil on missing ERC4626 config/call data instead of panicking in read path

Why

availableBalance is a withdrawable/health-constrained metric, not mark-to-market NAV. This keeps semantics explicit while preserving backward compatibility for non-ERC4626 strategies.