[Routine - VT] fix(commands): clamp bookmark position to document bounds in jumpToBookmark

[winterdrive]

Pre-flight Check

Open PRs / active branches checked before selecting this fix:

• [Routine - VT] fix(FileManager): use normalized membership check for add/remove files #79 routine/vt-fix-filemanager-relpath-260706 — touches src/core/FileManager.ts, src/test/unit/fileManagerRelpath.test.ts
• [Routine - VT] fix(provider): guard against stale groupIdx when rendering file bookmarks #78 routine/vt-fix-bookmark-stale-groupidx-260703 — touches src/provider.ts
• [Routine - VT] fix(extension): dispose treeView listeners and pending reveal timer #77 routine/vt-fix-treeview-listener-disposal-260702 — touches src/extension.ts
• chore: release v0.7.6 #76 release/0.7.6-260701 — touches CHANGELOG.md, package.json, package-lock.json

This PR only modifies src/commands.ts (the virtualTabs.jumpToBookmark command handler), which is not touched by any of the above. No functional or textual overlap.

Changes

virtualTabs.jumpToBookmark builds a vscode.Position directly from the stored bookmark.line / bookmark.character and immediately uses it for editor.selection and editor.revealRange. If the target file was edited outside VirtualTabs (e.g. lines removed) since the bookmark was recorded, the stored line/character can exceed the current document's bounds, so the editor ends up selecting/revealing an invalid position.

Fix: clamp the constructed Position with document.validatePosition(...) (the standard VS Code API for this) before building the range/selection, so the jump always lands on the closest valid position in the current document instead.

This PR does not:

• Add/rename/remove any command registrations, configuration keys, or dependencies.
• Modify package.json / package-lock.json / CHANGELOG.md.
• Touch tab-group serialization or storage/config logic.

Safety Verification

Commands run locally, both passed:

• npx tsc -p ./ — no errors
• npm run test (tsc -p ./ && jest --runInBand) — 25 suites / 175 tests passed

No lint or format:check script exists in package.json, so those steps were skipped.

CI / Release Gate Note

This is a daily routine Draft PR. Package version bump and CHANGELOG consolidation are intentionally deferred to the weekend release/integration PR. If CI fails solely due to the repository's version-bump/release-readiness gate, that is expected behavior for a routine PR, not a code validation failure.