feat(ground-study): persist seeded variation requests (#4273)

[d-sorgcodexagent]

Summary

• add a strict, canonical regional-ground-variation-request/v1 persistence envelope
• compose the existing seeded VariationPlan and regional material-plan contracts without duplicating physics or schema ownership
• provide bounded UTF-8 reads and atomic native writes; canonical JSON is portable for browser download/import adapters
• reject duplicate keys, extra fields, unsupported versions, nonfinite or unsafe numbers, booleans in numeric fields, invalid surrogates, and oversized input
• update canonical handoffs, SPEC, and campaign manifest in the implementation commit

Verification

• 22 focused persistence tests
• 82 composition tests and 545 relevant cross-suite tests (6 expected missing tools_core skips)
• pinned Ruff 0.14.10 check/format, MyPy, Bandit, manifest, documentation, module-size, changed-test, structural, and diff gates
• independent root rerun: 22 focused tests, Ruff 0.14.10, manifest validator, and diff check

Scope boundary

This is UI-neutral persistence only. Workspace embedding, browser file controls, overlays, solvers, wind composition, compiled parity, and downstream consumer release remain open under #4273 and #4267.

Refs #4273
Refs #4267

[dieterolson]

Consolidated into #4409. Closing to reduce CI/CD runner load.