[PaleRoses]

This removes the duplicate plans from cache/improved-plan. On master, the build cache holds every unit of the install plan three times: once in elaborated-plan and twice in improved-plan. The improved plan is the elaborated plan with units found in the store marked as installed.

improved-plan now holds the set of the plan's units that are in the store, keyed by the set of configured units. The store directory is monitored as before, and the marking is redone from the set on each run. An improved-plan written by an older cabal is treated as stale and overwritten without re-solving.

This is the follow-up I offered on #12463. It applies with or without #12464 and #12500. Each unit still keeps its own copy of the finalized PackageDescription, the follow-up already named in #12464.

I measured HLS at bc2bbb8e with the method from #12464, using GHC 9.12.4, -O1, assertions enabled and a populated dependency store. These are medians of six order-balanced cabal build all --dry-run runs on a shared macOS development machine. I applied the diff to 0687f8d2f. For the last column, I added one entry to the store between runs.

Version	improved-plan	whole cache/	No-op run	Run after the store changed
master 0687f8d2f	59.7 MB	93.5 MB	1.22 s	1.79 s
This PR	0.02 MB	33.8 MB	0.797 s	0.786 s
#12464 + #12500	22.9 MB	39.0 MB	0.419 s	0.610 s
#12464 + #12500 + this PR	0.02 MB	16.2 MB	0.250 s	0.252 s

The dry-run build list and plan.json were identical with and without the PR on every run, including after store additions and removals and toggling --disable-tests and back.

AI disclosure: written with Claude Code (Fable 5.2 and Opus 5.5).

---

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
  • Is the change significant? If so, remember to add significance: significant in the changelog file.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (StoreUnitRemoved)

QA Notes

Build a project with a dependency from Hackage; dist-newstyle/cache/improved-plan is a few KB. Build again; the second build is up to date. Delete the dependency's directory from the store; cabal build --dry-run lists it again. StoreUnitRemoved covers this.