Fix evolve state mismatch on evolution failure and preserve manual build rollback ability after AI change rollback

[Cas Linden (CasLinden)]

Summary

• Introduces manual_rollback_store_path to keep "Undo last build" available on ManualCommit across AI evolution/rollback cycles, independent of the AI rollback path
• Fixes post-AI-rollback routing back to ManualCommit by anchoring the rollback snapshot to the live nix path and a recorded changeset at adoption time
• Fixes restore_after_failure passing empty changes to state recomputation and leaving evolution-specific fields behind on fresh failure

Test Plan

Preserve rollback
Make a manual change, build it.
Try evolving, -> discard -> can still roll back build

Failed evolution
Make a manual change
Enter a prompt -> cancel before it finishes
Repo changes are restored, ~/Library/Application\ Support/com.darkmatter.nixmac/evolve-state.json -> reflects manual not evolve.
Caveat: this takes a watcher tick to fully resolve. Evolution errors do not return new evolve state object on failure. They pretend like nothing ever changed, which is good, but also why this fell through and was hard to find/fix.

Docs

• [] TODO: might add this to the manual test flow

[github-actions]

	Fails
🚫	This PR touches behavior-sensitive code that is documented in darkmatter/nixmac-web. Please either: Open a companion docs PR and check Docs updated in the PR description, or Check No docs update needed if the change doesn't affect user-facing behavior.

	Warnings
⚠️	Please assign this PR to someone (usually yourself).

📋 PR Overview

Lines changed	138 (+98 / -40)
Files	0 added, 9 modified, 0 deleted
Draft / WIP	no
Has Test Plan	yes
New UI components	no
New Storybook stories	no
New Rust modules	no
New TS source files	no
New tests	no
package.json touched	no
Cargo.toml touched	no
Infra / CI touched	no

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