nx_fx_lib.nx

nx_fx_lib.nx -- F984: EXACT MULTI-CURRENCY on the sovereign ledger. Every incumbent FX path loses fractions of a cent to rounding and buries it. The sovereign property here is that a conversion LOSES NO VALUE: the truncated remainder is RETURNED and accounted, never silently dropped, so a converted amount can be reconstructed EXACTLY. Money is neither created nor destroyed by crossing a currency boundary -- the same invariant the ledger enforces within one currency, extended across two. A rate is a RATIONAL num/den (integer, exact) -- never a float, because a float rate drifts and an auditor cannot reproduce it. A rate carries PROVENANCE (source + the pair it prices): a conversion can cite exactly which rate, from which source, produced its number. No provenance, no conversion is the honest posture, though the raw fx_convert is available for pure arithmetic. FAIL-CLOSED: a zero or negative denominator is REFUSED (return -1, remainder 0) -- a divide-by-zero rate is a data error, not a silent NaN. SCALE ENVELOPE (declared): fx_convert computes amount*num before dividing, so amount*num must fit i64 (~9.2e18). For cents and realistic rates this is billions of dollars of headroom; a 128-bit widening mul is the rung if a book ever needs trillions-of-minor-units single transactions. Declared, not hidden. DRY: composes nx_matter_lib (reg_put/reg_get/mt_field/canon_encode) for rate storage. license_tier: ORIGINAL LIB.