Reconcile package.json's engines.node floor with the actual runtime requirement

[bloknayrb]

Follow-up to #1442, which fixed tandem doctor (and the surrounding docs) to correctly report against package.json's declared "engines": { "node": ">=22.12.0" } floor instead of a stale major-only >= 22 check. That PR deliberately left one question open, because it's a product decision, not a bug fix.

The evidence, gathered while fixing #1442:

• Every package-lock.json entry whose engines.node mentions 22.12 is a dev: true devDependency — vite (^20.19.0 || >=22.12.0), rolldown and its per-platform @rolldown/binding-* packages (same range), @sveltejs/vite-plugin-svelte (^20.19 || ^22.12 || >=24). All 18 lockfile entries matching 22.12 were independently confirmed dev: true.
• The highest floor among genuine (non-dev) runtime dependencies entries in the lockfile is >=20.19.0 (dom-serializer).
• tsup.config.ts's target: "node22" (×3, server/channel/monitor entries) is an esbuild compile target, which means >=22.0.0 — not 22.12.0.
• Grepping src/ for Node APIs gated behind 22.12 (node:sqlite, fs.glob, styleText, process.features.require_module) returns nothing.
• npm's engines field is advisory by default (engine-strict off): a mismatch is an EBADENGINE warning, not an install-time block.
• src/cli/node-version.ts's CLI startup guard (nodeVersionError, gating every tandem/tandem-channel/monitor invocation, including the plugin's) already only enforces >=22.0.0 and was deliberately left that way in Node version floor is stated four ways; tandem doctor passes an install npm refuses #1442 — tightening it to 22.12.0 would have refused Node versions that run Tandem correctly today.

So 22.12.0 is real, but it's a build-toolchain floor (what a contributor's npm install + vite build needs), not the floor Tandem itself needs at runtime once built. package.json's single engines field doesn't distinguish the two, which is arguably one layer up from what #1442's symptom (doctor disagreeing with engines) was actually about.

The open question: should engines.node come down to >=22 (or >=20.19.0, matching the true runtime floor), with 22.12.0 kept as a documented contributor/build-only floor elsewhere (e.g. CONTRIBUTING.md, which already states it separately)? Or is a single conservative number simpler to maintain even though it overstates what's actually required?

Whatever the answer, tests/docs/node-floor-claims.test.ts (added in #1442) pins every prose copy of the floor to MIN_NODE_VERSION in src/cli/doctor.ts, so changing the number is a one-constant edit plus updating that test's expectation — the four docs and the code check can't drift apart silently again either way.

Not urgent — no user-facing bug results from leaving this open (the floor is currently the more conservative side of "safe").