docs(rust-ecosystem): correct sacn-unofficial date + freshness sweep

[IamMrCupp]

Closes #34.

Summary

Five corrections in RUST_ECOSYSTEM.md, all surfaced by the 2026-05-18 M1 scope-sharpening read (crates.io API + GitHub API + GHA runner docs all verified 2026-05-18).

• F1 (headline): sacn-unofficial last publish date corrected. The sACN section claimed "last published ~2024." crates.io API: last publish was 0.9.0 on 2020-11-30. The upstream Lan2u/RustSacn repo has had recent commits (last push 2026-05-11) — but no new crates.io release in 5+ years. New wording disambiguates "source touched" from "release shipped" and reframes the crate as a wire-format reference, not a runtime dependency candidate. Exactly the class of factual-error that PR docs(gui): lock Tauri (v2) and resolve framework-contingent annotations #31's freshness sweep caught two of (Iced sponsorship, Slint ambassador program).
• F2 (mirror of #33): macos-13 → macos-15-intel in the Cross-compilation section. GHA docs no longer list macos-13; current Intel macOS runner is macos-15-intel.
• F4: Last updated: line dated specifically — project planning phase, 2026. → 2026-05-18 (M1 scope-sharpening freshness sweep). The doc has been touched by PR docs: pivot deployment target from K8s container to native desktop app #29 + PR docs(gui): lock Tauri (v2) and resolve framework-contingent annotations #31 + this PR; the vague line didn't reflect that.
• F5 (sharpening): mlua Lua 5.5 one-liner. Current mlua (0.11.6) exposes a lua55 feature. Default stays lua54 for VJ-tooling alignment, but noted as revisit-at-M2.
• F6 (sharpening): artnet_protocol wording. Crate is 0.4.4 (2025-10-04), actively maintained — the "exists / vet quality" phrasing implied staleness. Reframed to "code-quality pass before adopting" so the action item stays valid without misleading on maintenance.

Source: working-folder research-m1-scope-sharpening.md. Touches the same feedback_verify_external_facts_before_locking.md discipline as PR #31 (private auto-memory).

Test plan

• CI green (markdownlint + lychee)
• GitHub render: sACN bullet's bold/date markup renders cleanly; the new wrap-points in the Cross-compilation paragraph don't introduce awkward line shapes