runtime(bun-compat): provide plugin / Bun.plugin as an inert registrar so OpenTUI's runtime-plugin support installs #10100

Parent: #10107. Depends on / pairs with: 01 (compile-time Solid JSX).

Problem

At TUI startup OpenCode runs ensureRuntimePluginSupport() (packages/opencode/src/plugin/tui/runtime.ts:44). That code does import { plugin as registerBunPlugin } from "bun" (node_modules/@opentui/solid/scripts/runtime-plugin-support-configure.js:1 and scripts/solid-plugin.js:1) and calls registerBunPlugin(createSolidTransformPlugin()) and registerBunPlugin(createRuntimePlugin(...)) (the @opentui/core/runtime-plugin specifier rewriter for user TUI plugins). Bun's plugin(p) invokes p.setup(build) synchronously; the plugin registers build.onLoad(...) loaders and records state.installed = true on globalThis[Symbol.for("opentui.solid.transform")].
Perry's bun module exposes stringWidth, file, write, stdin, hash, semver, JSONL, deepEquals, stripANSI, wrapAnsi, which, zstdDecompress*, gc, generateHeapSnapshot, Glob, pathToFileURL, fileURLToPath, TOML, YAML, spawn, … (crates/perry-hir/src/lower/expr_member.rs:340-360, crates/perry-runtime/src/bun_compat/) but no plugin; the member falls to js_bun_unsupported and the TUI dies before its first frame.

Scope

• Add plugin(p) to the bun module and Bun.plugin: accept { name, setup } (object) or a function; call setup(build) synchronously with an inert build object (onLoad, onResolve, onStart, onEnd, config, module are accepted and ignored); return undefined (or a resolved promise if setup returns one), matching Bun's shape. Also accept Bun.plugin.clearAll() as a no-op.
• Leave a diagnostic hook: when a later dynamic import would have needed a registered loader (a .tsx/.jsx user plugin), the deferred-import error should say the file type needs a runtime transform that the native build does not include (ties to decision DECISION: runtime-loaded JavaScript in the native OpenCode build — plugins, custom tools, non-bundled providers, TUI plugins #10105).
• Keep typeof Bun === "undefined" semantics for bundles that feature-detect Bun (member-position-only lowering, see bun-compat: Bun globals + "bun" module shim pack (Bun.stringWidth, Bun.file/write, Bun.stdin.text, Bun.hash) #6560).

Acceptance

• import { plugin } from "bun"; plugin({ name: "x", setup(b) { b.onLoad({ filter: /a/ }, () => ({})); } }) compiles and returns without throwing; Bun.plugin(fn) likewise.
• ensureRuntimePluginSupport() from @opentui/solid/runtime-plugin-support/configure returns true under perry and a second call returns false (installed-state bookkeeping works).
• The OpenCode TUI proceeds past src/plugin/tui/runtime.ts module init in the native binary.
• API manifest / docs drift gate updated (--print-api-manifest).