refactor: expose graph types as experimental API in fynd-core

[Haikane]

Summary

• Adds an experimental feature flag to fynd-core
• Re-exports only the specific types needed by downstream consumers (e.g. searchers) through a new fynd_core::experimental module, rather than making the entire graph or feed::events modules public
• Keeps graph and feed::events as pub(crate) internally

Exported types (behind experimental feature)

Graph: GraphManager, PetgraphStableDiGraphManager, StableDiGraph, EdgeData, EdgeIndex, Path, GraphError, EdgeWeightFromSimAndDerived, EdgeWeightUpdaterWithDerived

Events: MarketEvent, MarketEventHandler, EventError

Usage

[dependencies]
fynd-core = { version = "...", features = ["experimental"] }

use fynd_core::experimental::{GraphManager, StableDiGraph, MarketEventHandler};

Test plan

• cargo check -p fynd-core passes (no feature)
• cargo check -p fynd-core --features experimental passes
• CI passes