feat: fiber architecture rewrite with lane-based scheduler and type safety

[MarcelOlsen]

Overview

This PR replaces the old hook-based reconciler with a full fiber architecture modeled after React's internals. The public API surface remains compatible, but the internal reconciliation engine is completely rewritten.

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Architecture

Fiber Tree

• Work-in-progress / current dual tree for interruptible renders
• Fiber nodes track lanes (pending work), flags (side effects), and effects (hook lifecycle)

Work Loop

• performSyncWorkOnRoot — synchronous render + commit
• performConcurrentWorkOnRoot — time-sliced render with shouldYield checks
• ensureRootIsScheduled — routes sync lanes directly, non-sync lanes via scheduler

Begin / Complete Work

• beginWork — builds WIP fibers from element tree, calls component functions, reconciles children
• completeWork — finalizes host component props, creates DOM nodes, propagates effects upward

Commit Phase

• commitRoot — three-pass commit: mutation (DOM changes) → layout effects → passive effects flush
• commitWork.ts — DOM mutation helpers (create, insert, update, delete)
• commitRoot.ts — effect list traversal and execution order

Lane-Based Priority System

• 16-bit branded Lane / Lanes types with bitwise operations
• mergeLanes, intersectLanes, removeLanes, getHighestPriorityLane
• Lane entanglement for transition batching
• Priority levels: SyncLane, InputContinuousLane, DefaultLane, TransitionLane1/2, IdleLane

Scheduler

• Binary min-heap (O(log n)) for task queue instead of Array.sort()
• scheduleCallback, shouldYield, cancelCallback, runWithPriority
• lanesToSchedulerPriority / schedulerPriorityToLane bridge

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Type Safety Hardening

• src/fiber/bitwise.ts — centralised unsafe-cast boundary for all Lane / Lanes / Flags conversions. Zero stray as number elsewhere.
• Lanes = Lane alias — resolves impossible branded-type incompatibilities at every function boundary while keeping type branding.
• HookEffectTag = number — bitwise OR on effect tags naturally produces number; using a branded union would fail.
• Compile-time enforcement — tests/fiber/types.typecheck.ts with @ts-expect-error directives that fail on regression.

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New Modules (17 files)

Module	Purpose
src/fiber/bitwise.ts	Branded type conversion helpers
src/fiber/minHeap.ts	Binary min-heap for scheduler task queue
src/fiber/workLoop.ts	Work loop + sync + concurrent rendering paths
src/fiber/beginWork.ts	Begin work phase
src/fiber/completeWork.ts	Complete work phase
src/fiber/commitWork.ts	DOM mutation helpers
src/fiber/commitRoot.ts	Commit root orchestration
src/fiber/scheduler.ts	Priority task scheduler
src/fiber/lanes.ts	Lane priority system
src/fiber/childReconciler.ts	Child reconciliation with key-based diffing
src/fiber/fiberHooks.ts	Hook implementations for fiber architecture
src/fiber/effectList.ts	Effect collection and execution
src/fiber/workInProgress.ts	WIP tree management
src/fiber/createFiber.ts	Fiber factory functions
src/fiber/typeGuards.ts	Runtime type guards and assertions
src/fiber/hydration.ts	SSR hydration support
src/fiber/resumability.ts	Serialization / deserialization for SSR

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Removed Modules (7 files)

• src/dom-renderer/index.ts — replaced by fiber commit phase
• src/fragments/index.ts — inlined into core
• src/hooks/index.ts / src/hooks/types.ts — replaced by fiberHooks.ts
• src/reconciler/index.ts — replaced by full fiber reconciler
• tests/MiniReact.reconciler.test.ts — superseded by fiber tests
• .trae/rules/project_rules.md — stale project rules

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Browser Showcase

examples/interactive-showcase/ — Vite-powered live demo covering all 7 public APIs:

• useState — counter with history
• useEffect + useRef — interval timer with animated display
• useMemo + useCallback — filterable list with render count
• createContext + useContext — theme switcher
• createPortal — floating panel outside root DOM
• Fragment — dynamic grid columns
• useReducer — todo list with add/remove

Run with: cd examples/interactive-showcase && npx vite

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Test Coverage

• 473 tests, 0 failures, 1,526 expectations across 38 files
• 9 new test modules covering previously untested internals:
  • bitwise.test.ts, minHeap.test.ts, typeGuards.test.ts, lanes.test.ts
  • scheduler.test.ts, resumability.test.ts, hydration.test.ts
  • fiberUtils.test.ts, commitIntegration.test.ts
• Compile-time type enforcement: tests/fiber/types.typecheck.ts

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Breaking Changes

None for the public API. All internal refactoring. Existing consumers using:

import { createElement, render, useState, useEffect, useReducer, useRef, useMemo, useCallback, createContext, useContext, createPortal, Fragment } from "@marcelolsen/mini-react";

…will continue to work without changes.

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Verification

# Tests
bun test

# Type check
bun run typecheck   # 0 errors

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Checklist

• Tests pass (473 / 473)
• Type check clean (0 errors)
• Biome checks pass on new files
• Showcase example runs in browser
• No public API breakage

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Closes the entire fiber architecture stack (fiber/lanes, fiber/scheduler, fiber/workloop, fiber/public-api, fiber/tests, fiber/type-safety).