feat: core improvements

[subomi]

Summary

This PR introduces several core improvements to the requestmigrations library focused on performance, API clarity, and robustness.

1. Eager Graph Building (LR-2)

• Before: Type graphs were built lazily at request time, causing lock contention and unpredictable latency
• After: Graphs are now built eagerly at registration time with a lazy fallback for unregistered container types
• Uses sync.Map for thread-safe cache access with minimal lock contention
• No Finalize() step required—just register and use

2. API Surface Reduction (LR-3)

• Made TypeGraphBuilder unexported (typeGraphBuilder) - internal implementation detail
• Added cleaner request-scoped API: rm.For(req) / rm.Bind(req) returns a Migrator
• Simplified usage pattern: rm.For(req).Marshal(&data) / rm.For(req).Unmarshal(body, &data)

3. Native Type Migration Validation (ME-3)

• Added ErrNativeTypeMigration error for invalid registration attempts
• Blocks registration of built-in primitives: string, int, bool, float64, etc.
• Blocks unnamed composite types: []string, map[string]int, interface{}, error
• Allows user-defined named types: type MyString string, type User struct{}, type Tags []string

4. Concurrency Improvements

• Changed from sync.Mutex to sync.RWMutex for better read concurrency
• Added sync.Map for graph cache (lock-free reads)

5. Test Coverage Expansion (LR-5)

• Added comprehensive tests for complex types (nested structs, cycles, interfaces)
• Added Test_MigrationTypeValidation with 18 test cases covering type validation
• Added tests for eager graph building behavior
• Added tests for concurrent access patterns

6. CI Improvements

• Added -race flag to CI workflow for race condition detection

Files Changed

File	Changes
requestmigrations.go	+342/-304 - Core refactor with eager building, new API, validation
requestmigrations_test.go	+1134/-? - Expanded test coverage
common.go	+55 - Shared utilities
.github/workflows/go.yml	+3 - Race detector
util.go	-18 - Removed (consolidated)

Test Plan

• All existing tests pass
• New tests for type validation (18 cases)
• Tests for eager graph building
• Tests for concurrent access
• Race detector enabled in CI