Make Detekt a real gate, and record the convention-plugin form as ADR 0013

[simtop]

Two things: Detekt becomes a gate that can actually fail, and ADR 0013 records the convention-plugin form so it stops being re-litigated.

Detekt was not a gate

ignoreFailures = true meant it could not fail on anything, however bad — while AGENTS.md §5 advertises make lint as rung 5 of the verification ladder. It produced reports nobody was required to read. And source covered only src/main, so roughly a third of the repo's Kotlin — the part that decides whether the rest is correct — was never analysed at all, even in advisory mode.

Both are fixed the way the Android Lint gate here already works: the backlog present at adoption is grandfathered into per-module baselines, and a new finding fails the build. That preserves the property that matters — clearing the existing findings stays a batching decision, taken whenever it suits, rather than something that blocks work today.

Adoption cost: 36 findings in main sources, plus 13 more once test/androidTest are scanned, across 17 modules. What is being grandfathered, so it is a reviewed decision rather than a silent one:

module	rules
beer_data	CyclomaticComplexMethod, TooManyFunctions
beer_database	LongParameterList, MagicNumber, TooManyFunctions
beerdomain/fakes	TooManyFunctions, TopLevelPropertyNaming
core/designsystem	MagicNumber
feature/beerslist	LongParameterList
+ 12 more	mostly MagicNumber / TooManyFunctions in processors and test sources

Nothing was regenerated silently. Pre-existing baselines were restored first and refreshed only for modules that actually failed — the rule AGENTS.md states for lint-baseline.xml ("never by regenerating the baseline to bury a regression") applies identically here and is now written into the convention.

A bug that had to be fixed first

The extension sets baseline to null until the file exists. That is correct for the check task — a missing baseline should mean "no exemptions", not an error — but it left the task that creates the baseline with nowhere to write, so detektBaseline failed with property 'baseline' doesn't have a configured value on precisely the modules that needed one. The create task now gets the path unconditionally; it is an output, not an input.

Verified by mutation probe

A file tripping LongParameterList and MagicNumber:

> Task :core-common:detekt FAILED
LongParameterList - 7/6 - [The function tooMany(...) has too many parameters]

Removing it returns the build to green.

ADR 0013: convention plugins stay precompiled

I proposed converting them to binary plugins earlier this month. That was re-litigating a decision already taken deliberately — the project moved to precompiled scripts from binary plugins, for simplicity and maintenance — and my case was weaker than I presented it:

• The "you must go binary to share logic" argument is false. Precompiled script plugins can call top-level functions from a plain .kt in the same source set, which is how Versions.kt and AndroidCommon.kt already work. Both large reuse cleanups landed with no migration at all.
• The configuration-time argument leans on Now in Android's 12.7s → 0.76s, but this build has the configuration cache on, so that cost is not paid on a hit; and the measured numbers here (3.3s warm clean build, 1.6s incremental) are not consistent with a 12-second configuration phase anyway.

What survives is narrower and honest: the Gradle 9 accessor workaround would disappear, and build logic would become unit-testable — the only substantial untested part of a repo that enforces 13 invariants on everything else.

So the ADR keeps precompiled, and turns the comparison into a measurement task with a written method (convert one plugin, measure configuration time on a cache miss both ways, medians per ADR 0011) rather than a judgement call. It also records where Declarative Gradle sits: on top of binary plugins, not against them, and not before a stable release.

AGENTS.md's settled-decisions table points at it so the next reader does not re-propose it.

Verification

make test, make konsist, make lint, make screenshot-verify, spotlessCheck, plus the mutation probe above.