Scripting depth: runtime context, first-class arrays, dedicated scripting docs

[mbreissi]

Addresses three depth gaps called out in review — the v2 scripting/array work covered the simplest cases only. This makes each first-class.

1. Runtime context injected into the script scope

Every filter/script evaluation now binds, alongside the message view, the runtime context: thingName, componentName, componentFullName, routeId, and recvMs. A per-route ScriptContext is threaded app → pipeline → filter/script → RhaiEval. This lets a single generic script be reused across components and still stamp/branch on where it runs, instead of hard-coding identity.

2. First-class array-typed values

aggregate now folds an array value element-wise (recurse in Acc::fold_value), so an OPC UA array node / vector signal feeds avg/min/max/sum/count across its elements rather than being dropped as non-numeric. Array values already flowed through the [] filter/key spread, scripts (as Rhai arrays), and the file sink (JSON / explode) — now they aggregate too, and it's documented end to end.

3. Dedicated, substantive scripting documentation

New docs/scripting.md — its own page — replaces the thin section:

• the two forms; the full scope (message view + runtime context); return / error / statelessness semantics
• a Rhai language primer (functions, loops, ranges, switch, array map/filter/reduce)
• array handling
• a cookbook of 8 worked examples — derived units with a guard, array mean/peak/RMS, rate-of-change over samples, threshold-count filter, reusable identity-stamping, non-southbound normalization, switch status mapping, reduce — each explained goal → how → why, and each backed by a test in src/proc/script.rs.

explanation.md trimmed to an overview that links out; the reference scope table, a new "Handle array-valued signals" how-to, a data-types array note, and the docs index are updated.

Validation

• 39 tests pass (9 new scripting/array example tests, so every documented script is verified); clippy clean; default + minimal feature sets build.
• All internal doc links/anchors resolve.

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