refactor(env): share .nvmrc line parsing between reading and pinning

[liangmiQwQ]

The .nvmrc line rules (strip # comments, skip blank and key=value lines, reject multiple declarations) were written twice: once in read_nvmrc_file and once in nvmrc_version_span for vp env pin, with a comment asking them to stay in sync.

This PR moves those rules into a single nvmrc_version_spans iterator in vp_js_runtime that yields the byte range of each version declaration. Reading builds on it through a new pure parse_nvmrc_content, and pinning uses the same ranges to replace the version in place. The content cases in test_read_nvmrc_file now run against the pure parser without temp files.

No behavior changes.

🤖 Generated with Claude Code

[liangmiQwQ]

@codex review

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-19T15:13:09.771782Z	bbdb0ac	Manual request

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Chef's kiss.

Reviewed commit: bbdb0ac470

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