add FastProvideRoot and FastProvideWait options to UnixfsAddSettings,
allowing RPC clients to control immediate DHT providing of root CIDs
for faster content discovery

these options default to server config (Import.FastProvideRoot and
Import.FastProvideWait) when not explicitly set by the client

adds RPC client support for:
- ipfs provide stat (with --lan flag for dual DHT)
- ipfs dag import (with --fast-provide-root/--fast-provide-wait)

client/rpc changes:
- dag.go: add Import() method (~70 lines)
- dag_test.go: 4 test cases for Import (new file)
- routing.go: add ProvideStats() method (~25 lines)
- routing_test.go: 3 test cases for ProvideStats (new file)

to enable RPC client, refactored commands to use CoreAPI:
- add ProvideStats() to RoutingAPI interface and implementation
- add Import() to APIDagService interface and implementation
- commands delegate to CoreAPI (provide.go, dag/import.go)

address code review feedback for PR #11069:

- fix: propagate decode errors in client/rpc dag import (was silently dropping errors)
- fix: acquire pinlock before spawning goroutine to prevent race with GC
- fix: update fast-provide test to always expect failure in isolated environment
- test: add proper json compatibility test for provide stats (replaces compile-time check)
- docs: add educational comments explaining batch config defaults
- style: standardize error messages to use consistent "failed to X: %w" pattern

the pinlock fix is critical - moving acquisition before goroutine spawn prevents
blocks from being garbage collected before the lock is held. the error handling
fix ensures RPC clients receive decode errors instead of empty results.