Add ruler/measurement tool to viewer

[MatthewMckee4]

Summary

• Adds a ruler/measurement tool that lets the user click two points to measure the distance between them in world coordinates
• Distance displayed in appropriate units (nm, µm, mm) depending on magnitude
• Activated via R key or View → Ruler menu, cancelled with Escape
• Live preview line follows cursor after first click is placed

Closes #147

Test plan

• 11 new tests covering state machine (toggle, click workflow, cancel), Euclidean distance calculation, and distance formatting with inline snapshots
• All 106 viewer tests pass
• All pre-commit checks pass

[codecov]

Codecov Report

❌ Patch coverage is 52.61044% with 118 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/gdsr-viewer/src/ruler.rs	64.21%	73 Missing ⚠️
crates/gdsr-viewer/src/app.rs	0.00%	31 Missing ⚠️
crates/gdsr-viewer/src/viewport/mod.rs	0.00%	14 Missing ⚠️

Files with missing lines	Coverage Δ
crates/gdsr-viewer/src/main.rs	0.00% <ø> (ø)
crates/gdsr-viewer/src/viewport/mod.rs	50.50% <0.00%> (-1.44%)	⬇️
crates/gdsr-viewer/src/app.rs	0.00% <0.00%> (ø)
crates/gdsr-viewer/src/ruler.rs	64.21% <64.21%> (ø)

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[codspeed-hq]

Merging this PR will improve performance by 41.09%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚡ 1 improved benchmark
✅ 3 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	write_complex	2.3 ms	1.6 ms	+41.09%

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_{Comparing ruler-measurement-tool (05f5699) with main (327f160)}