feat(read): measured structural default summary (S1)

## Summary
Add a measured structural default-read summary for supported source files.

## Expected
Use a dependency-safe measured folder selection and one pure segmented-view renderer shared by both read surfaces, with exact source coordinates and safe raw fallback.

## Scope + acceptance criteria
- Measure heuristic and approved parser candidates against a fixed corpus and boundary manifest before selecting engines.
- Add one pure segmented-read renderer that preserves exact source lines and reports inclusive elided ranges.
- Use breadth-first structural unfolding within the approved visible-line budget and reject ambiguous syntax safely.
- Integrate the same folder and segmented view into both read tools without changing schemas or either truncation implementation.
- Summarize only eligible default reads; explicit offset or limit, prose, binary, unsupported, parse failure, no-summary, and truncation-boundary inputs retain today’s verbatim behavior.
- Reread elided ranges from fresh bytes using the existing offset/limit contract.
- Keep packaging within the approved embedded budget and retain explicit raw fallback where no safe engine is selected.

## Related
Plan rows 16-20 of the approved OMP adoption evaluation plan. Related structural/read and folding work: PRs #1460, #1500, #1490, #1324, and issue #494.

Start with the two existing read tools and the approved OMP adoption evaluation plan, especially rows 16-20; review related structural/read and folding work in PRs #1460, #1500, #1490, and #1324. Done means both read surfaces share the measured folder and segmented view, preserve schemas and truncation behavior, and retain safe raw fallback for all excluded cases.

Written by the indexing model from the issue text.