Fix malformed @@ header in formatHunkHeader fallback

[YuriNachos]

Summary

The fallback branch of formatHunkHeader now emits a well-formed unified-diff @@ header for every Hunk, including the edge case that previously produced a malformed one.

Root cause

The fallback used *Lines (changed-lines count) instead of *Count (total lines incl context) for the @@ -start,count +start,count @@ fields, so a Hunk with context lines got a malformed header.

Changes

• src/core/hunkHeader.ts — use *Count in the fallback @@ construction (matches the field semantics documented in @pierre/diffs + the existing primary branch).
• src/core/hunkHeader.test.ts (new, bun:test) — proves red→green: a fallback Hunk with deletionStart:10, deletionCount:4 now yields @@ -10,4 +10,4 @@ (was @@ -10,1 +10,1 @@), plus a pure-addition @@ -0,0 +1,3 @@ case and a context-suffix case.

Test plan

• bun test src/core/hunkHeader.test.ts — 3 pass, 0 fail.

This change was developed with AI assistance (Claude Code); every changed line was reviewed and understood.

[greptile-apps]

PR author is not in the allowed authors list.

[vercel]

@YuriNachos is attempting to deploy a commit to the Modem Team on Vercel.

A member of the Team first needs to authorize it.

[benvinegar]

The code change is correct, but this user-visible fix needs a patch Changeset for hunkdiff per the repository's release policy. The current branch fails:

bun run changeset:status --since=origin/main
# Some packages have been changed but no changesets were found.

Please add a .changeset/*.md entry with a concise user-facing summary.

This comment was generated by Pi using GPT-5.6 Sol

[YuriNachos]

Heads-up on the red check, so it doesn't read as a problem with this branch: the failing Vercel job is the fork-deployment authorization gate, not a build or test failure. Its status text is:

Authorization required to deploy.

and the target URL is vercel.com/git/authorize?team=Modem&…, i.e. a request for someone on the Modem team to authorize a deployment from a fork. It is red on every open fork PR in this repo right now, mine and other people's alike — sampled just now: #696, #695, #689 (mine) and #686, #682, #678, #676 (not mine), all with the identical message.

Every check a fork PR can actually influence is green here: Socket Security: Project Report and Socket Security: Pull Request Alerts both pass.

Nothing on this branch can turn Vercel green — it needs the authorize link clicked on your side. Happy to rebase or re-trigger if that helps.

[YuriNachos]

Added a patch changeset for hunkdiff in .changeset/clean-hunk-header.md. bun run changeset:status --since=origin/main now resolves cleanly:

🦋  - hunkdiff
🦋 info NO packages to be bumped at minor / major

(The red Vercel check is the fork-deployment authorization gate — not a build/test failure — as noted earlier; it is red on every open fork PR in this repo.)