docs(js): document Derived on the @moq/signals page

[kixelated]

Summary

• Adds the Derived section to doc/lib/js/@moq/signals.md: what it buys over Computed (sources named up front instead of tracked, a correct first read rather than undefined, no close()), what it costs (fn runs on every read, so it must stay cheap and pure), and the notify-on-actual-change behavior that lets a source move without moving the view.
• Also restores the two supporting mentions: the Overview line and the list of readables getter() accepts.

Why this was split from #2752

An adversarial review flagged the section in #2752: Derived does not exist on main or dev. It lives only in commit 3870cd1a on claude/js-origin-extract (#2705). #2752 therefore shipped without it, describing only what main exports, and the section was parked here.

Merging ahead of the implementation

This lands before #2705, which is the PR that actually adds the class, so the published page documents Derived ahead of the code. Until #2705 merges to dev and dev reaches main, a reader who copies the sample gets a failing import. Deliberate and self-correcting once #2705 propagates; flagged here so the gap is on the record rather than a surprise.

Public API changes

None. Documentation only.

Test plan

• Extracted all 31 fenced code blocks on the page and type-checked them under the repo's strict base config, with js/signals/src/index.ts taken from 3870cd1a so Derived resolves: zero errors. This confirms the new Derived([this.#peers], (peers) => peers.size > 0) sample infers correctly through the const S extends readonly Getter<unknown>[] generic and that peek() returns boolean.
• bun remark . --quiet --frail passes.
• just check on this branch cannot exercise the sample, since Derived is not in the tree here. Worth re-running once feat(js/net)!: route publish and consume through Origins, share one connection per relay #2705 lands.

(Written by Opus 5)

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• doc/lib/js/@moq/signals.md

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