perf: skip non-message preview lines

[sorafujitani]

Summary

• add a cheap "type" byte-level prefilter before preview JSON unmarshalling
• skip full decoding for lines that cannot be top-level user/assistant messages
• add coverage for spaced JSON, false-positive nested message-looking content, and a tool-heavy benchmark

Fixes #100

Benchmark

Command:

go test -bench=BenchmarkRenderTextToolHeavyTranscript -benchmem ./internal/preview -count=10

benchstat /tmp/ccsession-100-before.txt /tmp/ccsession-100-after.txt:

RenderTextToolHeavyTranscript-12  7.740m ± 2%  1.420m ± 3%  -81.65% (p=0.000 n=10)
RenderTextToolHeavyTranscript-12  5.906Mi ± 0%  2.898Mi ± 0%  -50.93% (p=0.000 n=10)
RenderTextToolHeavyTranscript-12  9.975k ± 0%  2.771k ± 0%  -72.22% (p=0.000 n=10)

Verification

go test ./...
go vet ./...
golangci-lint run
gofmt -l $(git ls-files '*.go')
git diff --check