docs(runtime): fix host-tag comment drift after tui- prefix rename

Adversarial review found stale bare-host-tag references the per-file sed couldn't
reach (they live in comments/docs). Code was clean — no contamination, no public
API leakage, root/text-leaf/comment asymmetry consistent. Updated:
- vite.config.ts isCustomElement comment (<box>/<text> -> <tui-box>/<tui-text>)
- component-authoring.md split-table Text row (virtual-text/text -> tui-*)
- box.vue / useBoxMetrics.ts / use-box-metrics.test.tsx "the real `box` host node"
  comments -> `tui-box`

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.agents/docs/component-authoring.md

@@ -19,7 +19,7 @@ not a reason to abandon the template. Two of the fixes also corrected latent inc
 | `Newline`   | template SFC                | `"\n".repeat(count)` interpolation + a context branch  |
 | `Static`    | template SFC                | reactive cursor + `v-for` scoped slot                  |
 | `Box`       | template SFC                | root `v-if` validation guard + `<slot/>`               |
-| `Text`      | template SFC                | `<slot/>` + `virtual-text`/`text` branch               |
+| `Text`      | template SFC                | `<slot/>` + `tui-virtual-text`/`tui-text` branch       |
 | `Transform` | **render function (`.ts`)** | inspects children: all-inert children → render nothing |
 
 ## Two questions, two idioms