Introduces the sfDust NUMBER field (id 18) on ltRIPPLE_STATE, a
sub-quantum residual for the trust-line credit path. Together with
sfBalance it forms the trust line's extended balance.
kSmdNeedsAsset is deliberately omitted: that flag rounds the field to
the asset's representable precision via associateAsset(), which is the
opposite of sfDust's purpose (it exists to hold value finer than that
precision).
No writer or consumer of the field yet; that lands in follow-ups.
Additive-only:
- New sfields.macro entry with rationale
- ltRIPPLE_STATE gains an SoeDefault sfDust slot
- Regenerated RippleState.h getters/setters + hasDust
- LedgerEntry.h.mako gains a conditional STNumber.h include so any
NUMBER-bearing ledger entry pulls STNumber directly (avoids
transitively relying on STLedgerEntry.h)
- RippleStateTests round-trip and nullopt coverage for sfDust
- .cspell.config.yaml picks up 'recognise*'/'renormalise*' used in
follow-up docs and code
