vopr: scenario + workload synthesis layers

[oreofeolurin]

What

First slice of the FloVOPR simulation-testing framework: src/vopr/{scenario,workload,mod}.zig — seed-deterministic swarm scenario synthesis (JSON-emitting, replayable via a serialized scenario rather than a bare seed) and self-verifying workload generation with the op oracle. stdx.PRNG exported; module registered in the unit-test root and as src.vopr.

Why

One seed must reproduce an entire cluster run — scenario, faults, every op payload. Payload/ring sampling is constrained so an AppendEntries batch can never evict its own head; deliberately-small rings live behind Scenario.smallRing.

Watch

• Reviewed (independent reviewer + comment auditor); folds: hardened the seed-differ test (was vacuous — the seed field differs by construction), the JSON test now parses output and checks field pairing, synthesize asserts its buffer/min contracts, comments scrubbed of references to not-yet-built machinery.
• The reviewer found the PRNG's own tests were never collected — root cause is repo-wide (no stdx-module test ever ran) and is fixed separately in test: compile stdx as its own test root — its tests had never run #65.
• Nothing in the server binary references src.vopr.
• 25 vopr tests; full unit suite 1341/1341 locally.