docs: record D-015 — FB3/FB4 arm64 tarballs are Android, not Linux

Per asfernandes in #38, the `.arm64.tar.gz`
assets for FB3/FB4 are Android builds with a misleading filename.
Linux ARM* packaging only starts at FB5.

Tightens the code comment at the `-ge 5` gate in `Update-Assets` and
adds a D-015 entry in DECISIONS.md so the rationale for the gate
lives in the customary place (the previous code comment framed this
as a fixable "packaging layout" inconsistency, which the upstream
answer ruled out).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: fdcastel

DECISIONS.md

@@ -91,3 +91,9 @@ Decisions made during the v2 rewrite, with rationale.
 **Decision:** The `generated/` directory is committed to the repository. After a successful publish, a workflow job (`update-repo`) regenerates the Dockerfiles and README and auto-commits any changes with `[skip ci]`.
 
 **Rationale:** README links to per-variant `Dockerfile`s (e.g. `generated/5.0.4/trixie/Dockerfile`) need valid targets on GitHub. Keeping the output in git guarantees the links work without requiring contributors to regenerate locally. Auto-commit prevents drift: the committed files always reflect the last successful publish. Contributors must not edit `generated/` by hand — run `Invoke-Build Prepare` to regenerate. The `[skip ci]` marker on the auto-commit prevents a trigger loop.
+
+## D-015: No ARM64 for Firebird 3.x / 4.x
+
+**Decision:** The ARM64 discovery gate in `Update-Assets` is `$majorVersion -ge 5`. FB3 and FB4 are published as amd64-only, even though `.arm64.tar.gz` assets exist in their GitHub releases.
+
+**Rationale:** The FB3/FB4 `.arm64.tar.gz` files are **Android builds with a misleading filename**, not Linux ARM64 — confirmed by asfernandes in [FirebirdSQL/firebird-docker#38](https://github.com/FirebirdSQL/firebird-docker/issues/38): "These are Android builds manually built by Alex. Names are very misleading. But better names were introduced in v5 only, as well Linux ARM* packages." Their internal layout reflects this (pre-extracted `firebird/` tree with `AfterUntar.sh`, no `install.sh`), which is why our Dockerfile template cannot consume them — but the deeper reason is that they are fundamentally the wrong binaries for a Linux container. No upstream packaging fix will change this for existing FB3/FB4 releases; any future "teach the template two install paths" workaround would install Android binaries on Linux and must be rejected. When FB3.x or FB4.x ships a future point release with a true Linux ARM64 bundle, the gate can be widened on a per-version basis.

firebird-docker.build.ps1

@@ -82,11 +82,9 @@ task Update-Assets {
                 }
             }
 
-            # Get arm64 release (FB5+ only). FB3 and FB4 ship arm64 tarballs but
-            # use a different layout (pre-extracted `firebird/` root with
-            # AfterUntar.sh instead of an `install.sh` installer), which our
-            # Dockerfile template can't consume — so we intentionally skip them
-            # and keep FB3/FB4 as amd64-only.
+            # Get arm64 release (FB5+ only). FB3/FB4 `.arm64.tar.gz` assets are
+            # Android builds with a misleading name (per asfernandes, FirebirdSQL/firebird-docker#38),
+            # not Linux ARM64 — Linux ARM* packaging starts at FB5. See DECISIONS.md D-015.
             if ($majorVersion -ge 5) {
                 try {
                     $arm64 = Find-FirebirdRelease -Version ([semver]"$version") -RuntimeIdentifier 'linux-arm64'