docs(readme): rework for user-facing clarity

[ShortArrow]

Summary

Addresses reader feedback that the README leans on internal jargon and internal architecture rather than the user's point of view. No functional change — docs only.

Changes

• Terminology clarified. "script" is now defined as a SCPI script file (.scpi); "scenario" as a query → response rule set. The overlapping record bullets are consolidated into run a mock / capture then replay / run & lint SCPI scripts, and visa script is surfaced for the first time (linked to ADR 0027). All features referenced are verified present (MockScenarioCommand record --from-script, VisaScriptCommand).
• use-first statefulness. The lead Highlights bullet now frames the current device like a shell's working directory: visa add registers, visa use selects, and later commands need no target at all.
• Connection architecture over internal architecture. The Clean Architecture layer diagram is replaced with a scenario-based Mermaid diagram (drive hardware / test without hardware / expose a gateway / control plane). Section retitled How it connects; the internal layering is kept as a one-line pointer to ADR 0003 / ADR 0021 for contributors.
• Samples labelled as test mocks. The Samples entry now states they are ready-made mock instruments for testing without hardware.
• Scalable language links. Inline per-doc ([日本語]) / ([English]) links are removed; only the top-of-file switcher remains. Recorded in ADR 0024 §4 (navigation links point to the English canonical only, so index lines don't grow per added language).

Both README.md and docs/README.jp.md updated in lock-step (i18n sync check).

Verification

• No stale #architecture anchors; ADR 0027 link resolves; only the two top switcher headers remain.
• csharpier clean; pre-push correctly detected docs-only and skipped build + test.