fix: correct README API examples, add tests, switch codecov to OIDC

[kolkov]

Summary

• Fix README: all code examples now use the actual API (New, Computed, Effect with explicit deps) instead of draft names (NewSignal, NewComputed, NewEffect with auto-tracking)
• Add 11 tests: computed cleanup/panic, readonly subscribe, Update+EqualFunc, effect default panic handlers, internal type erasure (float32, byte, struct, Signal[any], invalid deps)
• Switch Codecov to OIDC: tokenless auth via GitHub OIDC, no more CODECOV_TOKEN secret needed

Details

README fixes

What	Before	After
Constructor names	NewSignal, NewComputed, NewEffect	New, Computed, Effect
Dependency tracking	"Automatic tracking"	"Explicit dependencies"
Cleanup pattern	WithCleanup() + effect.Cleanup()	EffectWithCleanup(fn) + eff.Stop()
Stats	51 tests, 67.9% coverage	55 tests, 94.5% coverage, 28 benchmarks
Broken links	4 "coming soon" links to docs/guides/	Removed
ROADMAP link	docs/dev/ROADMAP.md	ROADMAP.md
Comparison table	"Angular-Compatible"	"Angular-Inspired"

New tests

File	Tests added
computed_test.go	Cleanup, Cleanup_Idempotent, SubscriberPanic, SubscriberPanicWithHandler
signal_test.go	Readonly_Subscribe, Update_WithEqualFunc
effect_test.go	CleanupPanic_DefaultHandler, PanicRecovery_DefaultHandler
internal_test.go (new)	NonCommonTypes (float32/byte/struct), InvalidDependency (nil/int/string), AnyTypeSignal

CI change

Codecov upload switched from token: ${{ secrets.CODECOV_TOKEN }} to use_oidc: true with id-token: write permission.

Test plan

• go test -v -cover ./... — 55 tests pass, 94.5% coverage
• go vet ./... — clean
• gofmt -l . — no output
• CI passes on GitHub (tests + codecov OIDC upload)

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