fix(tui): widen the root react range so an external ink can't split React

Review follow-up on #1983. Copilot was right, and about a hazard this repo
had already proven and then documented away: the claim that ink's ">=19"
peer "keeps npm from hoisting it next to a React the Inspector couldn't
also use" is false. A consumer pinning React 19.0 satisfies ">=19" while
`^19.2.4` nests underneath, so ink renders through their React and the
bundle through ours — the #1952 crash, one level up, breaking TUI startup
rather than just its forms.

Bundling ink closes it (verified: with a `createRequire` banner for the
inlined CJS, since react-reconciler calls `require("react")` and
signal-exit@3 `require("assert")`), but costs ~1.4MB — react-reconciler
and yoga-layout come along. The root `react` range closes it for free
instead: `^19.0.0` is open to the whole major, so npm can satisfy our
React and a consumer's pinned React 19 with one copy, and an external ink
resolves the same copy the bundle does.

Verified against both consumer trees — the reporter's (React 18 + ink 5)
and a pinned React 19.0.0 + ink 6.8.0, which splits under `^19.2.4`.

That makes the range load-bearing while looking like an ordinary version
bump, so the tests pin it: the root range must equal ink's own declared
peer floor. They also assert each exempt package is both external and a
root dependency — external means consumers install it, the mirror image
of the inlined ones, which must not be root-declared.

The docs in all four places now say ink is exempt on cost and never on a
peer range, and name that claim as one we got wrong.

Closes #1952

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WLYmVPVLdeYpE6aMTUsCks

package.json

@@ -93,7 +93,7 @@
     "ink": "^6.0.0",
     "open": "^10.2.0",
     "pino": "^9.14.0",
-    "react": "^19.2.4",
+    "react": "^19.0.0",
     "undici": "^8.5.0",
     "vite": "^8.1.5",
     "yaml": "^2.9.0",