Document Node 26 check invocation

[mgballou]

Summary

• Document the Node 26 flag needed for Vitest's jsdom environment to use its own localStorage implementation.

Verification

• pnpm install — passed
• NODE_OPTIONS=--no-experimental-webstorage pnpm check — passed (1,371 tests)
• pnpm build — passed
• pnpm harness — passed
• pnpm exec prettier --check README.md — passed

On Node 26, pnpm check without the flag fails because Node's experimental global localStorage masks jsdom's implementation.

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[mgballou]

Local verification: current and green

Main da92b00319bcbb46bfc074e9bb81a221f7c6c1b4 is already included; merging origin/main reported already up to date. Verified/pushed head: b9f9c31b951d39e03dafa8cb449e94037ccbe41a. HTTPS push reported everything up to date.

Node 26.7.0, pnpm 11.18.0, isolated worktree. PATH began with /opt/homebrew/bin; each check/build used /opt/homebrew/bin/gtimeout 900 rtk proxy.

Command	Exit	Result
pnpm install --frozen-lockfile	0	Locked dependencies installed
NODE_OPTIONS=--no-experimental-webstorage pnpm check	0	This PR's README command: typecheck/lint/format passed; 63 files / 1371 tests passed
pnpm build	0	Production build succeeded; no tests in this command

The documented Node 26 invocation works on this branch before #18 lands. Draft state unchanged. No PR merged.