web: add schematic back navigation

[yl12839-a11y]

Summary

Adds a "back" navigation control to the web schematic viewer so users can return to the previous view after drilling into a cell with double-click.

Note

• This PR is stacked on Feature/web schematic inverter symbol #10961. Diff will shrink to only the back-navigation changes once Feature/web schematic inverter symbol #10961 merges.

Type of Change

• New feature

Impact

Users can navigate back to the previous schematic view after double-clicking into a cell, instead of having to reload the schematic. Purely additive frontend behavior. No changes to placement, routing, timing, or any physical-design pipeline.

Verification

• [ x ] I have verified that the local build succeeds (./etc/Build.sh).
• [ x ] I have run the relevant tests and they pass.
• [ x ] My code follows the repository's formatting guidelines.
• [ x ] I have included tests to prevent regressions.
• [ x ] I have signed my commits (DCO).

Related Issues

Stacked on #10961.

[gemini-code-assist]

Code Review

This pull request enhances the schematic viewer by adding support for standard logic symbols (including DFF, DFFR, and DFFS registers), a back button with navigation history, double-click cell expansion, and improved label placement to prevent overlapping. The backend was updated to classify and map register pins, and the frontend now supports merging schematic cones and toggling between symbol and box views. The review feedback highlights a potential race condition when merging in-flight expansion requests and a performance issue (layout thrashing) caused by calling getBoundingClientRect() inside a sort comparator during label layout.

[gemini-code-assist]

There is a potential race condition here. If the user double-clicks a cell to expand it, a websocket request is sent. If the user performs another action (like a refresh or loading a different schematic) while the request is in flight, this._currentNetlist will be updated. When the expansion request completes, the new cone will be merged into the new netlist instead of the one where the double-click occurred. Using the captured previousSnapshot.netlist as the base for merging avoids this race condition.

                    const baseNetlist = previousSnapshot ? previousSnapshot.netlist : this._currentNetlist;\n                    const netlist = baseNetlist\n                        ? this._mergeSchematicNetlists(baseNetlist, data)\n                        : data;

[gemini-code-assist]

Calling getBoundingClientRect() inside the sort comparator causes layout thrashing (forced synchronous layout) because the browser is forced to recalculate styles and layout repeatedly during the sort operation (O(N log N) times). Pre-calculating and caching the bounding rectangles in a Map before sorting reduces the number of layout queries to exactly O(N), significantly improving rendering performance.

        const rects = new Map(records.map(r => [r, r.group.getBoundingClientRect()]));\n        records.sort((a, b) => {\n            const rectA = rects.get(a);\n            const rectB = rects.get(b);\n            return rectA.top - rectB.top || rectA.left - rectB.left;\n        });