runtime: match legacy print float exponents

[cpunion]

Summary

• Match Go legacy builtin print/println float exponent formatting by padding %e exponents to three digits for the !go1.26 runtime path.
• Add focused test/go coverage that compares baseline Go output with llgo output for builtin print/println, nil interfaces/maps/slices, integers, floats, complex values, bools, strings, spacing, deferred print calls, constant complex printing, and the generic type-switch float print shape from typeparam/typeswitch5.go.
• Remove the now-fixed print.go, deferprint.go, ken/cplx0.go, and typeparam/typeswitch5.go GOROOT xfail entries for go1.24/go1.25 on darwin/arm64 and linux/amd64.

goprint.go is intentionally not removed from xfail here. With the exponent format fixed, go1.25 still mismatches because go println(...) output can be truncated before the goroutine finishes; that is a goroutine/NumGoroutine scheduling issue, not a print/float formatting issue.

GOROOT CI is slow/disabled, so this PR keeps durable coverage in test/go. Normal PR CI should run from the fork branch.

Targeted Tests

• go test ./test/goroot -count=1 -run TestGoRootRunCases -args -goroot /Users/lijie/sdk/go1.24.11 -go /Users/lijie/sdk/go1.24.11/bin/go -dirs .,ken,typeparam -case '^(print|deferprint)\\.go$|^typeparam/typeswitch5\\.go$|^ken/cplx0\\.go$' -run-timeout 2m
• go test ./test/goroot -count=1 -run TestGoRootRunCases -args -goroot /Users/lijie/sdk/go1.25.0 -go /Users/lijie/sdk/go1.25.0/bin/go -dirs .,ken,typeparam -case '^(print|deferprint)\\.go$|^typeparam/typeswitch5\\.go$|^ken/cplx0\\.go$' -run-timeout 2m
• go test ./test/goroot -count=1 -run TestGoRootRunCases -args -goroot /Users/lijie/sdk/go1.26.0 -go /Users/lijie/sdk/go1.26.0/bin/go -dirs .,ken,typeparam -case '^(print|deferprint)\\.go$|^typeparam/typeswitch5\\.go$|^ken/cplx0\\.go$' -run-timeout 2m
• go test ./test/goroot -count=1 -run TestGoRootRunCases -args -goroot /Users/lijie/sdk/go1.25.0 -go /Users/lijie/sdk/go1.25.0/bin/go -dirs . -case '^goprint\\.go$' -xfail /tmp/llgo-empty-xfail.yaml -run-timeout 2m (expected mismatch; classified out of scope)
• go test ./test/go -count=1
• go test ./ssa ./cl ./runtime/internal/runtime -count=1 (./ssa and ./cl passed; ./runtime/internal/runtime is not in the main module)
• (cd runtime && go test ./internal/runtime ./abi ./internal/test -count=1)
• (cd runtime && go test ./internal/clite ./internal/clite/bitcast -count=1)
• git diff --check

[cpunion]

Root cause: the runtime change intentionally normalizes legacy builtin print/println float and complex output to Go's three-digit exponent form for pre-Go 1.26, but several fixed LLGo expectations still used the previous C %e two-digit exponent form. That broke cl fixture comparisons and the ESP32-C3 float-1664 startup regression check.

Fix: updated the affected fixed expectations (cl/_testgo, cl/_testrt, cl/_testdata, and _demo/embed/test_esp32c3_startup.sh) to the new legacy print format. I also expanded test/go/print_builtin_test.go to cover non-zero positive and negative exponents so this behavior is covered outside the slow goroot suite.

Local tests:

• go test ./cl -run 'TestRunAndTestFrom(Testgo|Testrt|Testdata)/(alias|invoke|multiret|struczero|complex|cpkgimp)$' -count=1 -v ✅\n- go test ./test/go -count=1 -v ✅\n- go test ./test -run TestBuiltinPrintLegacyFloatFormat -count=1 -v ✅\n\nPushed commit: 25ff4833525cc536709436d1db85d303a30698cc to cpunion/llgo:codex/goroot-print-coverage. New PR CI is running for this head; I will keep watching and iterate if anything fails.

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