Keep tour steps and code peeks clipped to their pinned lines

[jykim256]

Problem

When you step through a sequence diagram's tour, or open a code peek, the code card is supposed to show just that step's line range, like workflow_evaluation.py:199-204. Instead it shows the whole file for a newly added file, and the whole diffr hunk region for a modified file.

This broke in #502, "Preserve diffr context when applying review lenses." That PR made lensContextGaps() honor diffr's structural scope info (diff.contextScopes). That's the right call for the Diff tab, where you want a lens to keep the syntactic context around the code it highlights. But tour steps and code peeks use a different kind of lens. They build a "document" lens with one exact chunk in mind, and that lens goes through the same lensContextGaps() function.

The bug comes from the run-fill rule. When contextScopes is set, lensContextGaps() shows every unfolded run of lines that touches the lens range. For an added file, diffr folds nothing, so the whole file is one run. One small lens range anywhere in that file makes the whole file visible.

Fix

Added a new exact flag to ReviewDiffLens. The document lens (used by tour steps and code peeks) sets exact: true. When withLens() sees a lens with exact: true, it strips contextScopes off the diff before calling lensContextGaps(). Without contextScopes, only the old +/-3 line window around the pinned range applies, same as before #502.

Lenses used by the Diff tab never set exact, so nothing changes there. #502's scope-preserving behavior is still in effect for normal diff review.

Authoring guidance

• instructions/authoring.md now tells authors to pin the smallest range that shows the claim (usually 3-15 lines), since tour steps and peeks show only that range.

Reviewer notes

This touches the vendored Code - OSS fork (apps/review-desktop/code-oss/src/vs/review/**) and adds an optional field to ReviewDiffLens in review-protocol. No stored-data or telemetry changes.

Testing

Before left, After right

Added a test in reviewLens.test.ts that builds an added-file diff (300 lines, all inserted) with a scope covering the whole file, and a lens range of lines 80-87. It checks two things:

• Without stripping contextScopes (today's Preserve diffr context when applying review lenses #502 behavior), the whole file is visible — no gaps at all. This reproduces the bug.
• With contextScopes stripped (what exact: true triggers), only the +/-3 window around lines 80-87 is visible. This is the fix.

Ran the full test suite for apps/review-desktop:

• code-oss/src/vs/review/**/*.test.ts (204 tests via node --test) — all pass.
• scripts/**/*.test.mjs (120 tests) — 1 pre-existing failure (curated-extensions.test.mjs, "extracts nested Windows executables from a VSIX archive"), confirmed failing the same way on main before this change, unrelated to this fix (missing native module for VSIX extraction).

I did not run typecheck-client (tsgo over the whole code-oss client), because it needs the separate Electron-ABI npm install in code-oss/. The touched files compile under tsx in the test runs above. CI should cover the full typecheck.

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