feat(preview): annotate multi-bit pins with [N] width suffix

[jeffersonmourak]

Summary

Pin labels in --preview now render as name[N] for width > 1 and stay bare for width 1, so existing scalar preview goldens are unchanged. The annotation flows from the topology's per-component width byte through VirtualNode → PlacedComponent into the render pass.

Pin	Before	After
input a (width 1)	│ a │	│ a │
input[4] a	│ a │ (no annotation)	│ a[4] │
output[8] r	│ r │	│ r[8] │

Layout

pinSize now takes the signal width and adds widthAnnotationLen(width) to the padded box width (3 chars for single-digit widths, 4 for two-digit). The mixed-width fixture demonstrates scalar and multi-bit pins coexisting without truncation.

Subtle correctness fix

The Canvas.setCell stores slice pointers, not copies. My first cut composed the label in a stack-local [128]u8 = undefined buffer inside drawInputPin, which produced visibly garbled output (▢1+k instead of a[4]) once the stack frame popped — the canvas held dangling pointers.

Fix: compose the label in arena-owned memory at placement time. PlacedComponent gained a display_label: []const u8 = "" field; pin renderers fall back to placed.name when display_label is empty, so the dozens of existing test fixtures that build PlacedComponent literals don't need updating.

Test plan

• Four new preview fixtures cover the requested cases: multi-bit input pin label, multi-bit output pin label, multi-bit AND gate with width-4 inputs/output, mixed scalar + multi-bit pins.
• Existing scalar preview goldens are byte-identical (the bare-label path is unchanged).
• zig build test green from a clean cache.
• zig build bench reports golden matches on all 56 fixtures.

S11.2 (LED rendering for width >1 + --expand-display flag) is the matching follow-up that stacks on this PR.

Context: DOCS/plan-multi-bit-language.md §"Stage 11".

Closes #52