…idiffs (#2174)

## Summary

Fixes the manual-bump drift in the api-diff baseline (#2170) and makes
API changes visible in review.

- **Single source of truth.** The baseline lives only in the
`<api.diff.baseline.version>` pom property (bumped from the
already-drifted `1.5.1` to the actual latest release `1.6.1`).
`mise.toml` and `api-diff.yml` no longer hardcode it.
- **Renovate owns the bump.** A custom regex manager tracks the latest
*published* `io.prometheus:prometheus-metrics-core` on Maven Central and
bumps the property on the `renovate/api-diff-baseline` branch. Because
Renovate only proposes published versions, there is no Maven Central
propagation race and no post-release workflow or app token needed.
- **Diffs committed to `docs/apidiffs/`.** `mise run api-diff` syncs the
japicmp per-module reports into `docs/apidiffs/<module>.diff` via
`.github/scripts/sync-api-diffs.sh`, stripping the volatile preamble so
files only churn on real API changes. The api-diff workflow fails if
they are stale, so every API change shows up in the PR diff.
- **Regeneration on bump.** `generate-api-diff-baseline.yml` regenerates
`docs/apidiffs` on the Renovate branch and pushes it back, mirroring
`generate-protobuf.yml`.
- `docs/apidiffs/**` is marked `linguist-generated` so flint skips it.

## Notes

- The seed diffs are large because `1.6.1` predates `@StableApi` (the
bootstrap noise documented in #2168). They shrink to near-empty once a
release contains the annotations.
- Like the protobuf flow, a `GITHUB_TOKEN` push doesn't re-trigger CI —
close/reopen the Renovate bump PR to run the api-diff check after
regeneration (noted in the workflow).

## Validation

- `mise run api-diff` (generates the 25 seed diffs; verified idempotent)
- `mise run lint`
- `renovate-config-validator --strict`, actionlint, zizmor

Closes #2170

## Also fixes a pre-existing compat-test break on `main`

`micrometer-compatibility` and `jmx-exporter-compatibility` fail on
`main` (e.g.
[#2173](#2173)), unrelated
to this change. The compat harness installs local artifacts with
`-Dmaven.test.skip=true`, which skips building the `*:test-jar`
artifacts that the `activeByDefault` `default` profiles declare as test
dependencies, breaking resolution (e.g.
`prometheus-metrics-exposition-textformats:jar:tests`).

Fixed by deactivating those profiles in the compat install (`-P
!default`), matching what the release task already does (`-P
release,!default`). Verified locally: full `mvnw install
-Dmaven.test.skip=true -P !default` → BUILD SUCCESS.

## Reviewer note: the seed diffs show the full stable surface (expected)

The committed `docs/apidiffs/current_vs_latest/*.txt` list the
**entire** `@StableApi` surface as additions, not a small delta. This is
the documented bootstrap state from #2168, not a bug:

- The japicmp include filter is
`@io.prometheus.metrics.annotations.StableApi`. The baseline `1.6.1` jar
predates `@StableApi`, so its filtered surface is empty → every
annotated class in current code registers as "NEW". The header genuinely
compares against the `1.6.1` jar; the annotation filter is what makes
everything look added.
- Verified the filter is correct: only `@StableApi`-annotated types
appear (non-stable classes like `CKMSQuantiles`/`Buffer` are excluded;
`CallbackMetric` shows only as an inherited `NEW SUPERCLASS` reference).
It is the same root cause as the `breaking-api-change-accepted` label on
this PR.

It self-corrects after the next release ships `@StableApi`: Renovate
bumps the baseline to that release, the bump workflow regenerates
`current_vs_latest/` as an annotated-vs-annotated (near-empty) diff, and
the archived `<new>_vs_<old>/` becomes the first real release diff. So
the current files are best read as a one-time record of the initial
declared stable API surface.

---------

Signed-off-by: Gregor Zeitlinger <gregor.zeitlinger@grafana.com>