chore: update README for current state and add Codecov coverage

[LiranCohen]

Summary

• README overhaul: Updated to reflect the actual implementation state rather than the original design aspirations
• Codecov integration: Added coverage profile generation and upload to CI

README changes

Before	After
"Design phase"	"Pre-release" with accurate status
did:dht references	did:jwk (what we actually use)
"dexnet" networking engine	"meshnet" (our actual fork)
100.64.0.x IPs	10.200.x.x (our actual CIDR)
Two protocols (wireguard-mesh + wireguard-node)	Single wireguard-mesh protocol
meshd init + meshd network create quick start	meshd up --create (what actually works)
Outdated project structure	Accurate directory listing with all packages
No CLI reference	Full CLI command listing
No protocol structure	Protocol hierarchy diagram
No development instructions	Build/test commands

Codecov

• Added -coverprofile=coverage.out -covermode=atomic to the unit test step
• Added codecov/codecov-action@v5 upload step with fail_ci_if_error: false
• Added Codecov badge to README
• Public repo, so tokenless upload should work (may need repo registration on codecov.io)

Verification

• go build ./... -- zero errors
• go vet ./... -- zero warnings
• go test ./... -count=1 -race -coverprofile=coverage.out -- all tests pass, coverage generated