fix(globals): reject -r 0 to match must-be-positive contract (#71)

[TimD1]

Summary

Globals::parse_args (src/globals.cpp) validated the -r / --max-ram value with if (this->max_ram < 0), but the error message reads "Max RAM must be positive". Message and logic disagreed: -r 0 was silently accepted despite the message claiming the value must be positive.

How max_ram is used downstream

I checked whether 0 is a meaningful sentinel (e.g. "unlimited") before choosing a fix. It is not:

• src/globals.cpp:439 — ram_steps.push_back(this->max_ram / threads); with max_ram == 0 every RAM step is 0.
• src/dist.cpp:790 — while (total_ram < g.max_ram && thread_step >= 0); total_ram starts at 0, so 0 < 0 is false and the "not enough work for all threads" branch schedules zero threads, silently dropping superclusters.
• src/cluster.cpp:607 — if (mem_gb > g.max_ram); with max_ram == 0 every supercluster "exceeds" RAM and triggers the warning path.

There is no "0 means unlimited" semantics anywhere; 0 simply breaks scheduling.

Fix

Changed the check to if (this->max_ram <= 0) so the validation matches the "must be positive" contract stated in the message. This is the minimal change and the option consistent with actual usage (rather than softening the message, since 0 is not a valid value).

Compile check

cd src && g++ -c -g -O1 -Wall -Wextra -std=c++17 globals.cpp -o /tmp/probe_71.o — clean, no warnings.

Fixes #71
Sub-issue of #45; documented in docs/D2_vcfdist-v3-unit-tests.md §6 defect #13.