Add serverFunctions.components: 'external' for hosts that own the document wiring - #341

Fixes #340

serverFunctions.components now accepts 'external' alongside booleans. It behaves exactly like true everywhere (same transforms, same codegen — every derived serverComponents flag is truthiness-based, so 'external' flows as enabled), but declares that a composing host (the Astro adapter, TanStack Start's Solid integration) owns the document wiring — render plugin + client-side installServerComponents() call — itself, so the configResolved without-SSR-start-mode warning is skipped. Under full SSR start mode it's redundant but harmless: treated exactly as true, no redundancy warning.

Also fixes the stale warning text for the remaining true path: it still listed "the bootstrap script" as a required app-side piece, but head bootstrap injection was removed (serialized server-component references self-bootstrap the registry). The message now names the render plugin (with the direct-call transform) and installServerComponents(), and points hosts at components: 'external'.

• src/server-functions/index.ts: widen the option type, refresh the JSDoc
• src/index.ts: skip the warning on 'external', reword the message
• README.md: document 'external' in the server-components section (reuses the external vocabulary, cf. start.external)
• .changeset/server-components-external.md: patch changeset
• examples/start-ssr/test/components-warning.mjs (chained into the suite's test script): config-level assertions that true warns with the updated text (no "bootstrap" mention, mentions 'external'), 'external' is silent yet still enabled (serve-time pre-bundle of the server-components runtime matches true), off stays off, and 'external' under SSR start mode is silent

Local gate: pnpm build (rollup + tsc) green, node test/components-warning.mjs 9/9, node test/run.mjs frames 61/61.

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