Buy an OUI guide: fix JSON example, add release install, tighten copy

[jthiller]

What changed

Follow-up cleanup of docs/network-iot/run-an-lns/buy-an-oui.mdx after #2086 pointed purchases at iot@helium.com. One file, ~117 insertions / ~86 deletions.

Substantive

• JSON example: the env info output had a JSX comment ({/* env vars will be discussed later */}) inside the ```json fence, which rendered literally and left invalid JSON. Replaced with the real environment block the CLI prints (all `unset` at this stage, verified against `src/cmds/env.rs`), plus a pointer to the Configure Routing Rules section where those variables get set.
• Remaining hedges: "Direct purchase ... is not currently available on-chain" is gone, as are the last two places that named the Helium Foundation as the party selling devAddrs. Factual Foundation references (NetID assignment, dashboards) stay.
• Install from release: the CLI ships prebuilt binaries (v0.2.4, macOS arm64 / Linux x86-64 / Windows). Added a "Running From a Release" path first; the cargo build is now "Building From Source".
• Purchase checklist: the wallet address line now says legacy format (what step 2 produces) and that it becomes the payer key; the devAddr line says slabs of 8 and links to the sizing guidance.
• Key pairs: Owner bullet now precedes Delegate (which referenced the owner key before it was defined); delegate guidance deep-links to Add and Remove Delegate Keys.
• Pricing: states the $135 minimum for own-NetID operators, and that the DC minimum is a balance the operator funds rather than a fee paid to Helium. Own-NetID alternative is stated once with a roaming link instead of three times.
• Community section: both bullets now say what to do (PRs against helium/well-known), with the LNS Providers eligibility criteria from that repo's README.
• Diagram: routing hop relabelled from "OpenLNS" to "Helium Packet Router".
• Smaller copy fixes: chatty "let's put a copy in the local bin folder" paragraph, MIC-overhead justification, "more devAddrs can be allocated" now says by whom.

Consistency polish

devAddr casing (frontmatter, table header, mermaid labels), heading case, code-block title punctuation, "(Mac)" → "(macOS)", trailing slash on the Fund an OUI link, footnote period placement, quote style, explicit anchor ids on linked headings, "blocks of 8" → "slabs of 8", NwkID 6b'111100 → 0b111100, NwkSKey introduced before the diagram uses it.

Why

A multi-lens audit of the page after the email swap turned these up. Full list was verified item-by-item; the applied set excludes items that didn't hold up (e.g. the LoRaWAN 1.0.3 spec link was reported dead but is live and unchanged).

Notes for reviewers

• yarn build passes (onBrokenLinks: 'throw'). On the built page every new anchor resolves, the mermaid diagram renders with the new subgraph, and there are no console errors.
• Prettier passes.
• Deliberately not changed: sidebar labels (the sibling "Fund an Organizationally Unique Identifier" label is hardcoded in sidebarsDocs.js, so shortening one without the other would look odd), and no Session Key Filter cross-link was added to the multiplexing section.
• The one remaining "currently" (legacy key format, line 72) describes a real technical constraint rather than a promised change, so it stays.

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