feat: read Tron assets from controller with fallback

[ulissesferreira]

Explanation

Stacked on: ##360 (WPN-1497-migration-foundation)

This is PR 2/4 for WPN-1478.

Current state: The Snap reads assets from local state exclusively. Asset queries (balance checks, fee computation, send validation) never touch the AssetsController.

This PR: Routes all 11 asset read sites through getAssetForStage(), a 4-stage switch that encodes the full rollout:

• Stage 0 (Off): local state only (current behavior)
• Stage 1 (ReadAssetsControllerWithFallback): controller first, fall back to Snap state
• Stage 2 (ReadAssetsControllerWithoutFallback): controller only, throw if missing
• Stage 3 (ReadAssetsControllerOnly): same as stage 2

Also introduces buildControllerSnapshot() for initial sync and gates KeyringEvents on stage < 3 (events fire through stage 2 for rollback safety, per the Jira spec).

Files: 8 modified. +411 lines.

References

• Parent: WPN-1478 (Migrate Tron Snap to consume AssetsController)
• Sub-task: WPN-1497 (Stage 0→1)
• WPN-1498 (Stage 1→2, config-only advance) behavior already encoded in the stage switch
• Depends on Core: WPN-1473, WPN-1489, WPN-1490

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

[ulissesferreira]

Tentative approach under old ASSETS-2 shape — superseded by WPN-1478 redesign.