engine: deep-copy ReversibleMeta in ResCopy

[karpfediem]

Fixes a reversible-resource copy bug caused by aliasing *ReversibleMeta.

engine.ResCopy() was reusing the same reversible metadata pointer for the original resource and its copied/reversed form.
Reversal setup mutates that metadata (Disabled, Reversal), so those mutations could leak back into the original live resource.

MRE

First run

file "/tmp/reversible-meta-copy-mre/" {
      state => $const.res.file.state.exists,
}

# Step 1 of the runtime proof for the reversible meta aliasing bug.
#
# Run this file first, let it converge, and then run
# `test/shell/reversible-meta-copy-mre-cleanup.mcl` with the same `--prefix`.
#
# Example:
#   ./mgmt run --prefix /tmp/reversible-meta-copy-mre-prefix \
#     --converged-timeout=2 --no-pgp lang \
#     test/shell/reversible-meta-copy-mre.mcl
#
#   ./mgmt run --prefix /tmp/reversible-meta-copy-mre-prefix \
#     --converged-timeout=2 --no-pgp lang \
#     test/shell/reversible-meta-copy-mre-cleanup.mcl
#
# Expected on a correct engine:
# 1. This run creates `/tmp/reversible-meta-copy-mre/target`.
# 2. The cleanup run logs `adding 1 reversals...` and removes the target.
#
# Buggy engine behavior:
# 1. This run converges and then deletes its own pending reverse state during
#    shutdown cleanup.
# 2. The cleanup run has no stored reversal to add, so the target is left
#    behind.
file "/tmp/reversible-meta-copy-mre/target" {
      state => $const.res.file.state.exists,
      content => "proof\n",

      Meta:reverse => true,
}

Second run

file "/tmp/reversible-meta-copy-mre/" {
      state => $const.res.file.state.exists,
}

Impact

This could cause the original resource to be treated as if it were itself a reversal resource during cleanup, which in turn could delete the pending reverse state that should have been preserved for a later run.

In practice, that means a normal converged reversible resource can disappear from the graph, and the next run has
nothing left to reverse.

Fix

Add (*ReversibleMeta).Copy() and use it in engine.ResCopy() so reversible metadata is copied by value instead of
shared by pointer.

Verification

Added focused Go tests covering:

• copy isolation for ReversibleMeta
• Reversed() not mutating the original resource
• ReversalInit() not mutating the original resource
• ReversalCleanup() not deleting the original resource's pending reverse file

The MCL repro is a second commit, so feel free to cherry-pick.

[purpleidea]

This comment is so suspicious! haha

I will dig a bit further into this patch this week, thank you!

[purpleidea]

Sorry for my delay here...

This run converges and then deletes its own pending reverse state during shutdown cleanup.

It should be deleting the created file on clean shutdown with ^C. Is that not occurring or it's not what you expected?