Fix dev supervisor readiness after UTF-8 log output

[hanafusay]

Summary

• treat readiness log offsets as byte offsets
• preserve readiness detection after earlier UTF-8 output
• cover the behavior through the real child process lifecycle

Problem

The supervisor records a log position using the file size in bytes, but readiness polling applied that value as a JavaScript string character offset. Earlier multibyte UTF-8 output could therefore cause a newly appended readiness marker to be skipped.

Verification

• node --test test/dev-supervisor-child.test.ts
• npm run typecheck
• targeted ESLint and Prettier checks

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[time-attack]

Thanks for this, @hanafusay — and sorry for the slow response.

We're closing this PR. Per CONTRIBUTING.md, since coding agents write most of the underlying code now, we'd rather receive feature ideas as a short human-written note in adrs/ — informal is great, just the idea in your own words — and bugs as a plain issue. If we're aligned we're happy to burn our own tokens on the implementation, and we'll credit you as co-author.

We don't want your idea to get lost, so we've captured it in #835 to triage on our side. Please feel free to open an adrs/ note if you feel strongly about the approach. Appreciate you engaging with the repo!