Delete sdk/mpr — the legacy engine is gone

[ako]

Deletes sdk/mpr. 163 files, 41,674 lines. This is the last step of
docs/plans/2026-09-14-retire-legacy-engine.md, whose porting slices are already
upstream — this branch is those two commits and nothing else, on top of current
main.

Why it can land on its own

The importer count was taken from 27 to 0 by the slices already merged here. On
this base the only remaining mention of the package is the guard test that held
the count at zero, and the six importers of the sdk/mpr/version subpackage.
Both are handled in the first commit.

Zero importers was not the same as safe to rm -rf

Two live dependencies survived the count reaching zero, and neither is visible
to a check written against the parent package's import path:

dependency	why an importer census misses it
sdk/mpr/version — 6 importers, two of them shipping code (docker/build.go, patch.go)	a subpackage is a different import path
sdk/mpr/testdata/v1-project — read by docker/update_widgets_test.go	an os.DirFS string, not an import at all

Before deleting a package, search for three things: its own import path, its
subpackages' paths, and its directory as a literal string (testdata,
go:embed, scripts). The last two are invisible to any importer census.

The six went to mdl/types, not to modelsdk/mpr/version, which looks like
the natural home and is the wrong one:

// sdk/mpr/version — an ALIAS, so types.ProjectVersion is the same type
type ProjectVersion = types.ProjectVersion

// modelsdk/mpr/version — a DUPLICATE STRUCT, which would be a different type
type ProjectVersion struct { ... }

Reading the declaration rather than the name is what distinguishes them. The v1
fixture moved to modelsdk/mpr/testdata/.

Everything was repointed and proven green with the package still present,
then deleted as a separate step — which is what separates "the repoint was wrong"
from "the deletion was wrong".

Two measurements worth keeping

• The shipped binary is identical in size before and after, so the linker had
  already dropped the package: this removes source weight, not runtime
  behaviour. docker check and check --post-migration are byte-identical to
  the pre-deletion binary.
• sdk/widgets fell to zero importers as a side effect but is deliberately
  kept — modelsdk/widgets/dirty_template_test.go reads
  sdk/widgets/templates/mendix-11.6 by path. The same path-not-import trap,
  caught by grepping the directory name. Worth its own look; not this PR's call.

The import guard is removed

The guard test that held the count at zero is deleted with the package: an import
of something that does not exist is a compile error, which is strictly
stronger than a test asserting the same thing.

The second commit: CLAUDE.md routing

Deleting the package left the always-in-context file pointing at files that are
gone — the architecture tree described the deleted engine and omitted modelsdk
entirely, the BSON storage-name procedure sent you to
sdk/mpr/parser_microflow.go, and the test-first checklist sent a parser test to
sdk/mpr/. Eight sites updated; every path the file now names was checked to
exist. Two checklist rules that were about the dead package are restated as
rules about the engine that does exist.

Docs under docs/ are deliberately untouched: they describe what the code used
to do, and ADRs are immutable by convention.

Verification

Run on this branch, on top of current upstream main:

gate	result
make build	pass
go test ./...	pass
cherry-pick onto this base	clean, no conflicts

And on the fork before merge: make vet (incl. -tags integration),
the full repo-wide make test-integration (exit 0, no failures), make check-mdl
(560), make check-findings (1,091 records)

[github-actions]

AI Code Review

Critical Issues

• None found

Moderate Issues

• None found

Minor Issues

• None found

What Looks Good

• The PR correctly deletes the entire sdk/mpr legacy engine package as the final step in retiring it, following the verified porting slices already merged.
• All necessary dependency updates were handled: sdk/mpr/version aliases moved to mdl/types, test fixtures relocated to modelsdk/mpr/testdata/, and literal string references (like in docker/update_widgets_test.go) were addressed.
• The import guard test was removed with the package, turning a test assertion into a compile-time safety.
• CLAUDE.md was thoroughly updated to remove references to the deleted package and correct the architecture tree/checklist.
• Verification shows zero runtime impact (binary size identical) and all tests pass (make build, go test ./..., make test-integration, etc.).
• The two commits are appropriately scoped: first handles code/repointing, second updates documentation.
• No MDL syntax or features were modified, so related checklist sections are not applicable.
• The PR adheres to the project's deletion protocol by verifying three things before removal: import paths, subpackage paths, and literal string directory references.

Recommendation
Approve the PR. The deletion is safe, complete, and well-verified. No changes are needed.

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