Sprint S.1: Extractor internal seams (Phase S)

[randlee]

Summary

Sprint S.1 of Phase S hotspot-remediation (issue #572, docs/plans/phase-S.md). Reduces complexity in the JSON/YAML/XML extractors (crates/sc-composer/src/extract/{yaml,json,xml}.rs) via private-only seam extraction. Closes S-T1, S-T2, S-T3.

Bottom layer of Phase S's linear gh-stack: integrate/phase-s -> S.1 -> S.2 -> ... -> S.8.

Test plan

• cargo fmt --all --check
• cargo clippy -p sc-composer --all-targets --all-features -- -D warnings
• cargo test -p sc-composer
• cargo test --workspace
• just lint
• git diff --check
• quality-mgr sprint_review QA (rust-qa-agent, req-qa, arch-qa, simplification-reviewer)

Sprint doc: docs/phase-S/sprint-s-1-extractor-internal-seams.md

[randlee]

@/private/tmp/claude-501/-Users-randlee-Documents-github-sc-compose/8398afb7-9b28-472d-bc40-ac36cb0f83fa/scratchpad/qa-pr576-sprint-s1-report.md

[randlee]

@/private/tmp/claude-501/-Users-randlee-Documents-github-sc-compose/8398afb7-9b28-472d-bc40-ac36cb0f83fa/scratchpad/qa-pr576-sprint-s1-r2-report.md