fix(web): keep an assignment's row in place on write

[graphics80]

Summary

editAssignment, setAssignmentLock, and setAssignmentClosed rebuilt <classroom>/assignments.json as "every entry except this slug, then the updated entry", so touching an assignment moved its row to the end of the array. They now go through a small replaceAssignmentEntry helper that replaces the row where it sits.

That matches the CLI, which is the other side of this contract and writes the rule down: UpsertAssignment ("Position preserved on replace; new slugs append", assignments_json.go) replaces the first match in place, and gh teacher assignment lock assigns back into the same index. It also matches the web app's own renameAssignment, which already maps in place; deleteAssignment is a plain filter and createAssignment/copyAssignment append new slugs, both already correct.

What the old shape cost teachers:

• gh teacher assignment list prints entries in file order (no sort), so locking hw1 in the web app moved it below hw9 in the terminal and in --json output.
• A one-field flag flip committed as a whole-row delete + re-add, so git log -p assignments.json no longer showed what actually changed.

No reader is affected: every consumer looks entries up by slug, so this is an ordering and diff-hygiene fix, not a behavior change for students or grading.

The helper replaces the first match only, like UpsertAssignment: a hand-edited manifest can hold two rows for one slug, and each caller builds its entry from the first of them, so replacing both would overwrite the second row's own content. (The old filter+push silently collapsed such duplicates into one row, dropping the second row's content; keeping both matches the CLI and preserves the teacher's data.)

Also updates the comment at rename.ts:286, which cited "the lock flip's filter+push" as the counter-example for mapping in place. That pattern no longer exists after this change, and the comment now names createEdit.ts and UpsertAssignment rather than a symbol that isn't in that file (addresses the Copilot review thread).

Tests

Each of the three writers gets a regression test asserting the surrounding rows keep their positions, plus one for the duplicate-slug manifest. All three position tests fail on main (the old code produces [hw2, hw1] for edit and [hw0, hw2, hw1] for lock/close against the new fixtures). Adding neighbor rows to the shared lock/close fixtures does not weaken any existing assertion in those blocks.

Merged main

Rebased onto current main via merge (102 commits, including #827 team assignments, #839 lock-from-form, #771 repo_visibility, and #855, all of which touched createEdit.ts). No textual conflicts. Re-verified after the merge that:

• all three call sites still route through replaceAssignmentEntry, and no writer added on main reintroduced filter+push (deleteAssignment remains the only .filter on slug, as intended);
• each caller still resolves the entry with find and throws before reaching the helper, so the miss branch stays unreachable;
• the ref-update path (updateRef with force: false) is unchanged, so the concurrent web/CLI write window is identical to before.

Closes #754

Type of change

• Bug fix
• New feature
• Documentation
• Refactor / maintenance

Checklist

• I built, tested, and linted the module(s) I touched: cd web && npm run check (web only, no Go or Python change). Re-run on the merged tree: tsc, eslint, prettier, arch:validate clean; 5192 unit tests pass, including all 253 in assignments.test.ts.
• If I changed a cross-binary contract, I updated schemas/*.schema.json and every mirror (Go / Python / TypeScript), and the parity tests pass. n/a: no schema or field change; this aligns the web app with the CLI's existing write behavior.
• If I added or changed a CLI command or flag, I documented it in the wiki (not in a README). n/a: no CLI change.
• My commits follow Conventional Commits.

[Copilot]

Pull request overview

This PR fixes an ordering/diff-hygiene issue in the web app’s writers for <classroom>/assignments.json by updating existing assignment entries in-place rather than removing and re-appending them (which previously moved the touched row to the end of the array). This aligns web write behavior with the CLI contract (“Position preserved on replace; new slugs append”) and improves the auditability of config repo diffs and CLI list output order.

Changes:

• Added a replaceAssignmentEntry helper to replace the first matching assignment row by slug while preserving array position.
• Updated editAssignment, setAssignmentLock, and setAssignmentClosed to use the helper instead of filter + push.
• Added regression tests ensuring row position is preserved (and that duplicate-slug manifests only rewrite the first match), plus updated a related comment in rename.ts.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
web/src/domain/assignments/createEdit.ts	Introduces replaceAssignmentEntry and uses it in the three affected writers to preserve row position.
web/src/domain/assignments/rename.ts	Updates an explanatory comment to remove the now-stale “filter+push” reference.
web/src/domain/assignments.test.ts	Adds regression tests covering position preservation for edit/lock/close, plus a duplicate-slug scenario.

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[rongxin-liu]

Thanks!