fix(#1858): replace emoji map icon with Phosphor sprite in rx-coverage (#1860)

## Summary

`public/rx-coverage.js` still carried a literal `🗺️` (U+1F5FA) in the
Mobile RX coverage page header — missed by the #1648 emoji → Phosphor
migration. Replaced with `ph-map-trifold` from the existing sprite,
matching how every other page header renders (`analytics.js`, `home.js`,
`node-analytics.js`, `customize-v2.js`).

```diff
-'<h2 style="margin:4px 0 2px;font-size:18px">🗺️ Mobile RX coverage</h2>' +
+'<h2 style="margin:4px 0 2px;font-size:18px"><svg class="ph-icon" aria-hidden="true"><use href="/icons/phosphor-sprite.svg#ph-map-trifold"/></svg> Mobile RX coverage</h2>' +
```

## How it got through

This is not a gap in the tooling — `test-issue-1648-m6-final-sweep.js`
catches it correctly. On current `master` (`a06ac8ac`):

```
✗ 1 emoji-as-icon violation(s):

  public/rx-coverage.js:29 [U+1F5FA] '<h2 style="margin:4px 0 2px;font-size:18px">🗺️ Mobile RX coverage</h2>' +
```

The gate is in `test-all.sh` but not in the CI test list in
`deploy.yml`, so it never runs. That divergence is filed separately as
#1858 — this PR is the concrete defect it let through.

## Test plan

- [x] `node test-issue-1648-m6-final-sweep.js` — `✓ lint gate: 0
violations across public/** and cmd/**` (was 1 violation before)
- [x] `node test-issue-1648-m6-lint-self.js` — green, including the
anti-tautology probe (it requires a clean repo to run at all, so it was
failing on master purely as a cascade from the above)
- [x] `eslint public/rx-coverage.js` — 0 errors (1 pre-existing
`no-unused-vars` warning on `selectedName`, untouched)
- [x] `ph-map-trifold` confirmed present in
`public/icons/phosphor-sprite.svg`

Single-line change, no behaviour change beyond the icon glyph.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: SaarMesh-Bot <300107934+SaarMesh-Bot@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>

public/rx-coverage.js

@@ -26,7 +26,7 @@
 
   function pageHtml() {
     return '<div style="max-width:1100px;margin:0 auto;padding:12px 16px">' +
-      '<h2 style="margin:4px 0 2px;font-size:18px">🗺️ Mobile RX coverage</h2>' +
+      '<h2 style="margin:4px 0 2px;font-size:18px"><svg class="ph-icon" aria-hidden="true"><use href="/icons/phosphor-sprite.svg#ph-map-trifold"/></svg> Mobile RX coverage</h2>' +
       '<div style="color:var(--text-muted);font-size:11px">Where roaming CoreScope-RX clients heard nodes. Colour = best signal per cell. <a href="https://github.com/efiten/corescope-rx" target="_blank" rel="noopener">Get the companion app →</a></div>' +
       '<div class="analytics-time-range" id="rxDays" style="margin:8px 0">' + dayBtn(1) + dayBtn(7) + dayBtn(14) + dayBtn(30) + '</div>' +
       '<div class="nq-cov-legend"><span><i style="background:var(--nq-cov-strong)"></i>strong</span><span><i style="background:var(--nq-cov-mid)"></i>medium</span><span><i style="background:var(--nq-cov-weak)"></i>weak</span><span><i style="background:var(--nq-cov-grey)"></i>no signal</span></div>' +