fix(release-script): install root artifact before ShowcaseSync

[DemchaAV]

Summary

• cut-release.ps1 Run-ShowcaseSync now runs ./mvnw -B -ntp -DskipTests install -pl . immediately before exec:java, in both Release and PostReleaseOnly modes.
• Step 0 pre-flight branch / clean / sync gates are relaxed for -DryRun so the script can be previewed from a feature branch while iterating on it. Live cuts still fail the gates loudly.
• Lesson captured in docs/contributing/release-process.md section 4 alongside the v1.5.0 / v1.6.0 / v1.6.5-prep entries.

Why

During the v1.6.5 cut Step 4 (ShowcaseSync) aborted with:

Could not find artifact io.github.demchaav:graphcompose:jar:1.6.5 in central

Sequence of events that produced it:

1. Step 1 bumped the four pom.xml files (root, aggregator, inherited examples, inherited benchmarks) to 1.6.5.
2. Step 4 invoked mvnw -f examples/pom.xml exec:java ... to regenerate docs/examples.json.
3. The examples module's dependency on graphcompose:${project.version} resolved to 1.6.5.
4. Local ~/.m2 only had 1.6.4 (the previous release) — there was no install gate between Step 1 and Step 4.
5. Maven failed dependency resolution; ShowcaseSync exit 1; the script threw.

The cut had to finish by hand: mvnw install, re-run ShowcaseSync, verify, commit, tag, push. Same gap will fire on every future cut on a maintainer machine whose ~/.m2 doesn't already contain the new version (clean checkout, CI, fresh laptop). The install is cheap (~15-20 s) and idempotent.

What changes

• Run-ShowcaseSync: prepend the install call. Both the live invocation and the dry-run preview emit it.
• Step 0 (pre-flight): wrap the branch / clean / origin-sync checks in a -DryRun guard so dry-runs work from any branch. Live cuts still see all three gates.
• release-process.md section 4: new "v1.6.5 cut" lesson entry.

Verification

Dry-run from this feature branch shows both Maven invocations in the right order:

[3] Switch ShowcaseMetadata GH_BASE to /blob/v1.6.6-test
    [DRY RUN] ShowcaseMetadata GH_BASE -> /blob/v1.6.6-test

[4] Regenerate docs/examples.json with v1.6.6-test links
    [DRY RUN] mvnw.cmd -B -ntp -DskipTests install -pl .
    [DRY RUN] mvnw.cmd -f examples/pom.xml exec:java "-Dexec.mainClass=com.demcha.examples.support.ShowcaseSync"

[5] Run mvnw verify (sanity check)
    [DRY RUN] mvnw.cmd verify -pl .

Pre-flight gate output confirms relaxation only in dry-run:

[0] Pre-flight checks
    branch: fix/cut-release-install-root-before-showcasesync (gate relaxed for -DryRun)

Test plan

• Dry-run from this feature branch shows install step before ShowcaseSync
• Dry-run from this feature branch shows pre-flight relaxation note
• Reviewer skim of the script diff — fix is localized to Run-ShowcaseSync and Step 0
• Next live cut (v1.6.6 Maven Central debut) exercises the fix end-to-end — this PR is the gate before that work starts