Add opt-in SourcePawn 1 compatibility mode

[rumblefrog]

Summary

• add a use-old-spcomp input for SourceMod 1.13.7451+ while retaining upstream SourcePawn 2 as the default
• preserve the SourcePawn 2 executables and isolate compiler/proxy cache variants
• make the Linux proxy safe for quoted paths and arguments
• cover SourcePawn 1 and 2 selection on Linux and SourcePawn 1 compatibility on Windows

Validation

• npm run build
• tsc --noEmit
• real SourceMod 1.13.7451 SP1/SP2 fixture compilation
• end-to-end proxy, no-proxy, quoted-path, and cache-reuse checks
• Windows archive layout and transformation checks

Fixes #40

[Mikusch]

What's the advantage of adding use-old-spcomp rather than asking users to explicitly switch to oldspcomp in their workflows? Users who rely on version: '1.13.x' without pinning a specific version should expect potential breaks. Since a workflow change is necessary regardless, wouldn't it be better to add docs on how to use the legacy compiler instead of adding a setting that silently renames binaries?