[compiler] Add experimental structured hooks lowering prototype

[luckysolanki902]

Summary

• add an experimental enableEmitStructuredHooks compiler flag and 'use structured hooks' directive path
• lower a tiny structured subset of conditional useState() / useMemo() calls into experimental_useStructuredHooks(...)
• add compiler fixtures for successful lowering and explicit rejection of unsupported hook placements
• add a compiler integration test proving the lowered form preserves dormant branch state across hide/show rerenders
• fix the snapshot harness so fixture @target pragmas actually control runtime-module selection
• document the current experimental surface and its deliberately narrow limits

Why

This explores a different runtime representation for hook identity.

Today, the Rules of Hooks are enforced by a cursor-based hook model where state is identified by call order. This experiment asks whether a compiler can carve out a narrow opt-in subset where hook identity is instead assigned explicitly with stable keys.

The branch stays intentionally small:

• only functions opting into enableEmitStructuredHooks
• only functions using 'use structured hooks'
• only direct variable initializers for useState() and inline useMemo() with literal dependency arrays
• explicit errors for unsupported hooks, unsupported placements, and non-component / non-custom-hook functions

That keeps the claim tight: this does not pretend arbitrary conditional hooks are solved. It demonstrates a concrete lowering seam where branch-local hook state can survive disappear/reappear cycles while still collapsing to one real top-level hook call.

Validation

• yarn workspace react-compiler-runtime test
• yarn workspace babel-plugin-react-compiler run jest parseConfigPragma-test.ts structuredHooksTransform-test.ts
• yarn workspace babel-plugin-react-compiler lint
• yarn workspace babel-plugin-react-compiler test