feat(installer): add stable C:\Ruby4Lich5\current junction to versioned Ruby tree

[mrhoribu]

What

Creates a stable C:\Ruby4Lich5\current directory junction that points at the
versioned Ruby tree laid down this install (e.g. C:\Ruby4Lich5\4.0.5), so
front-ends can hardcode C:\Ruby4Lich5\current\bin instead of a path that moves
every time the baked Ruby version changes.

Why

Some front-end clients have to hardcode the Ruby bin location in their prefs.
Today that's C:\Ruby4Lich5\<rubyver>\bin, which changes on every Ruby bump and
forces every affected front-end to reconfigure. A stable current\bin gives them
one path that survives version changes.

How

Three additive edits, all in [Code]:

• CurStepChanged var line extended with LinkPath, RubyDir.
• A new rubygem-gated block in CurStepChanged (runs alongside, not inside, the
  existing lich block so it also fires for a rubyonly install). It clears any
  prior current, then creates the junction.
• A new CurUninstallStepChanged that removes current on uninstall so it can't
  dangle after the versioned tree is gone.

Junction, not symbolic link

Uses mklink /J (directory junction), not mklink /D (symlink), deliberately.
A /D symlink needs SeCreateSymbolicLinkPrivilege, which a standard user does
not have unless Developer Mode is on — and this installer is
PrivilegesRequired=lowest, so a symlink would silently fail for many users. A
junction needs no elevation and works for a local, same-volume, absolute-path
redirect, which is exactly this case (both ends under C:\Ruby4Lich5). Front-ends
see current\bin\ruby.exe transparently.

Non-fatal by design

Ruby is already fully installed and usable at the versioned path before the
junction is attempted. A junction hiccup (AV interference, a pre-existing
non-empty current) logs a warning and continues rather than aborting an
otherwise-good install. The pre-clear uses plain rmdir (no /s), which removes
only a junction's reparse point — never the target's files — and fails safe on a
real non-empty directory instead of deleting it.

Scope / non-goals

PATH, file associations, and the registry entries are intentionally left pointed
at the versioned path ({app}\{#RubyVersion}\bin\...). Repointing those at
current is a larger change with its own failure modes (e.g. uninstalling one
version breaking PATH) and would be a separate discussion. This PR only adds the
convenience path the front-ends asked for.

Behavioral decisions worth a look

• current = last installed. The junction always follows the most recently
  installed Ruby. If older versioned dirs linger, current still resolves to the
  newest. This matches the motivation but is the one real behavioral contract here.
• Pre-existing real current dir. Handled fail-safe (no data loss, junction
  just skipped that run). Chose this over rmdir /s.

Testing

No automated harness exists for a .iss, so this was reviewed by reading against
jrsoftware/issrc step ordering. Suggested manual matrix on a Windows box:

1. Fresh full install -> C:\Ruby4Lich5\current\bin\ruby.exe -v resolves.
2. Re-install a bumped Ruby version -> current now points at the new tree, old
   junction gone.
3. rubyonly install -> current is still created.
4. lichonly install -> no junction created, no error.
5. Uninstall -> current removed, no unrelated files touched.

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✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch feat/add-junction-ruby-path

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[github-actions]

Proposed next installer version

R4L5-v040.002.004

Compared against the latest installer release (R4L5-v040.002.003):

• CI 3 -> 4 (installer/workflow files changed since R4L5-v040.002.003)

Review and bump installer-version in a real dispatch of build-ruby4lich5-installer.yaml to 040.002.004 when this PR is ready to ship as a release. This is a proposal only -- nothing is assigned automatically.