compiler(bun): suppress unknown-identifier warnings for the provided Bun global

[proggeramlug]

Problem

Bun-platform mode now provides the real globalThis.Bun namespace (#9599), and bare Bun resolves to it at runtime. The HIR diagnostic path still reports every bare use as an unknown identifier:

Warning: unknown identifier 'Bun' — assuming global; resolved by name on globalThis ...

Large Bun-target graphs can emit dozens of these false-positive warnings, obscuring actionable compatibility diagnostics.

Minimal repro

console.log(Bun === globalThis.Bun);
console.log(Bun.version);
console.log(typeof Bun.file);

perry compile main.js --platform bun -o app

The compile should not warn that Bun is unknown.

Acceptance criteria

• In explicit Bun-platform mode, an unshadowed bare Bun is classified as a platform-provided global and emits no unknown-identifier warning.
• It still lowers through the same runtime namespace introduced by bun-compat: add an opt-in Bun platform mode with a real globalThis.Bun namespace #9599.
• A lexical binding named Bun shadows the platform global normally.
• Non-Bun/default platform mode keeps its existing feature-detection and diagnostic behavior.
• Other unknown identifiers are unaffected.
• Tests cover bare reads, typeof Bun, computed access, destructuring, and shadowing in both platform modes.

This issue is diagnostic hygiene only; it does not add new Bun namespace members.

[proggeramlug]

Fixed by #9754, which landed on main via merge train #9798. The train was merged as its own branch, so the Fixes #9745 keyword in #9754 never fired — closing manually. Verified the fix is on main by patch-id (git cherry), not just by the PR being closed.