[RWong04]

Adds a restore control beside the language picker that puts the current language's starter back, as accepted in #348.

The starter goes in through insertText over just the span that differs, so one Ctrl+Z restores the previous buffer. A browser without that command, or one that refuses it, would have to assign the buffer and leave nothing to undo; there the first press leaves the code alone and only arms the control, and a second press confirms. Chromium redoes a replaced selection that ends in a newline wrongly, so for the restore only, that newline is removed by a forward delete in the same undo step. Undo does not survive a language switch, which holds for any edit and is #365's subject. The control is disabled while paused and in the behavioral round, and does nothing once coding is closed.

The reset is an ordinary code_update, so no message type, recording kind or contract version changes. A Test credit already recorded survives it: reconcile_received_test (src/agent.rs:2776) appends a Test row only while test_recordable (src/agent.rs:2171) holds, and nothing removes a recorded row, since evict_one_observation (src/agent.rs:1515) never takes a phase's only one. A reset before the run is recorded stops it, so the candidate must write and run again.

Screenshots, from a page with no interview running: an edited buffer, the code after one press, after Ctrl+Z, the control dimmed while paused, a 390 px wide screen, and the prompt shown by a browser without insertText

Validation: restore-starter.test.js adds 52 tests, and each deliberate breakage I tried fails at least one. ./scripts/test.sh passes with 1599 browser and 1886 Rust tests; skipped locally are commentflow, actionlint and the Linux-only Clang fetch. I also restored the starter in every offered language of all 151 problems (747 cases): one press restores, and undo and redo are exact. A live LiveKit and Gemini session was run on an earlier revision, where the first press armed, and not repeated.

Limits: the toolbar wraps to a second row at 590 to 620 px, and the icon-only control relies on its tooltip and aria-label. In Playwright's WebKit a restore joins the typing before it, so one Cmd+Z undoes both. Safari's click-without-focus behaviour is modelled in a test, not tried in real Safari. Firefox could not be launched here, so it is unchecked whether it records the insert and the forward delete as one undo step.

Closes #348

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Summary by cubic

Adds a Restore starter button to the editor toolbar so a candidate can put the current language's starter code back and start over. The reset goes through the browser's editing command over just the differing span, so one Ctrl+Z (Cmd+Z on Mac) restores the previous buffer; a trailing newline is handled separately to avoid a Chromium redo bug. In browsers without a working editing command, the first press only arms the control and a second press confirms, both in the toolbar. Only the current language's buffer changes, the reset sends as a normal code_update, and the button is inert while paused, in the behavioral round, or after coding closes. Closes #348.

^{Written for commit 07c4e56. Summary will update on new commits.}

[jserv]

Please fold the six commits into one when you address these; the folded message should drop the sentence from f3a0673 saying a browser that refuses the command asks before assigning value, since eae3bf3 reversed it.

web/interview.html:210: This comment still says the toolbar stays one row, but the PR body notes it now wraps at 750 to 780 px and 1130 to 1160 px, and the restore button sits in the display-settings group although it edits code. Either move the button next to the language tabs or update the comment.

[jserv]

Rebase latest main branch and resolve conflicts.

[jserv]

Thank @RWong04 for contributing!