Phase 1: PS7 correctness fixes

[exactmike]

Summary

Phase 1 of plans/modernization-roadmap.md: low-risk PS7 correctness fixes, each reproduced and verified against a real GraphViz install before/after the fix.

• Export-PSGraph -showgraph and destination path problem KevinMarquette/PSGraph#110 / PR Replace Invoke-Expression with Invoke-Item KevinMarquette/PSGraph#102 — -ShowGraph failed on a destination path with spaces (Invoke-Expression splits on spaces). Swapped in Invoke-Item. Confirmed the dot.exe argument-array invocation itself already handles spaced paths correctly on PS7 via splatting, so no --% stop-parsing token is needed there.
• PSGraph in Azure Pipelines  KevinMarquette/PSGraph#97 / Problem with chars KevinMarquette/PSGraph#104 — BOM breaks dot's parser: reproduced by setting $OutputEncoding to UTF8-with-BOM (as some CI/profile environments do), which injected a BOM into the stdin pipe to dot and caused syntax error near 'digraph'. Fixed by pinning $OutputEncoding to UTF8-no-BOM inside Export-PSGraph, independent of caller/ambient state. Verified non-ASCII labels already round-trip correctly through the pipe once the BOM is gone.
• Make the paths to dot dynamic KevinMarquette/PSGraph#75 / Detect graphViz outside of -GraphVizPath KevinMarquette/PSGraph#88 / Doesn't detect Graphviz on linux KevinMarquette/PSGraph#85 — GraphViz path detection was a hardcoded path glob only. Now prefers a cross-platform Get-Command dot PATH lookup when -GraphVizPath isn't explicitly supplied, falling back to the glob; an explicit -GraphVizPath still wins outright rather than silently falling back to PATH.
• Setting compound=false fails KevinMarquette/PSGraph#98 — Graph's begin block unconditionally set compound = 'true', silently overriding a user-supplied compound=$false. Now only defaults it when the caller hasn't already supplied a value.
• SubGraph Example from help doesn't work KevinMarquette/PSGraph#66 — the documented SubGraph -Attributes @{...} { } example doesn't bind ScriptBlock (a real PowerShell positional-binding limit across the 4 parameter sets — restructuring positions to support it breaks basic positional binding elsewhere). Fixed the doc to the working call shape (-Attributes @{...} -ScriptBlock { }) instead, per the roadmap's fallback option.

Also removed the now-dead PSAvoidUsingInvokeExpression suppression on Export-PSGraph, since no Invoke-Expression call remains in the module.

Test plan

• Added regression tests in Tests/Regression.Tests.ps1 (compound, SubGraph binding) and Tests/Export-PSGraph.Tests.ps1 (spaced path, ShowGraph, BOM, non-ASCII, path detection)
• Full local Pester run: 145 passed / 9 failed, all 9 pre-existing and unrelated (3 need a built .psm1 not present in this checkout — Phase 2 CI work; 2 are pre-existing ScriptAnalyzer findings in Entity.ps1/Node.ps1 untouched by this PR)
• PSScriptAnalyzer clean on all three changed source files
• Manually verified each fix against a real local GraphViz install (dot - graphviz version 15.1.1) before and after the change

Claude-Session: https://claude.ai/code/session_01AgrjdhkmUCswsFGbn3EwCg