Fix arch-dependent integer overflow in gage_validate for single-point timeseries

[Copilot]

When a rain gage timeseries has only one data point, table_validate() never updates dxMin from its initial sentinel value of BIG (1×10¹⁰). Casting BIG * SECperDAY (~8.64×10¹⁴) to int overflows — negatively on x86-64 (silently passes the check) but positively on arm64 (spuriously triggers WARN09 about recording interval).

Change

• src/solver/gage.c — gage_validate(): Guard the gageInterval computation and interval comparison checks behind if ( Tseries[k].dxMin < BIG ). A single-point timeseries has no meaningful minimum interval, so the checks are correctly skipped rather than relying on undefined overflow behavior.

// Before: always computed, overflows for single-point timeseries
gageInterval = (int)(floor(Tseries[k].dxMin*SECperDAY + 0.5));
if ( gageInterval > 0 && Gage[j].rainInterval > gageInterval ) { ... }
if ( Gage[j].rainInterval < gageInterval ) { /* spurious WARN09 on arm64 */ }

// After: skip entirely when dxMin was never updated from BIG
if ( Tseries[k].dxMin < BIG )
{
    gageInterval = (int)(floor(Tseries[k].dxMin*SECperDAY + 0.5));
    if ( gageInterval > 0 && Gage[j].rainInterval > gageInterval ) { ... }
    if ( Gage[j].rainInterval < gageInterval ) { ... }
}

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