debian+makefile: expose POSTIT_RUNTIME override for arm64 builds

Default stays linux-x64 (the only architecture built today).
Override on the command line to target arm64 (or any other
RID supported by Avalonia 12 on Linux):

  make deb POSTIT_RUNTIME=linux-arm64

The Makefile passes POSTIT_RUNTIME through to dpkg-buildpackage,
which propagates it to debian/rules, which threads it into
dotnet publish --runtime.

Note: the .deb is mono-architecture. For a multi-arch .deb,
the build must run twice (once per RID) and the resulting .debs
can be combined with dpkg-gencontrol / Multi-Arch: same.

Update README accordingly.

Makefile

@@ -13,16 +13,18 @@
 # Environment overrides (or pass on the make command line):
 #   POSTIT_GIT_URL   default: https://github.com/pazof/yavsc.git
 #   POSTIT_GIT_TAG   default: 1.0.0
+#   POSTIT_RUNTIME   default: linux-x64  (override to linux-arm64 etc.)
 #   POSTIT_OUT_DIR   default: $(CURDIR)/..    (artifacts land here)
 
 POSTIT_GIT_URL ?= https://github.com/pazof/yavsc.git
 POSTIT_GIT_TAG ?= 1.0.0
+POSTIT_RUNTIME ?= linux-x64
 POSTIT_OUT_DIR ?= $(CURDIR)/..
 
 .PHONY: deb clean source
 
 deb:
-	POSTIT_GIT_URL=$(POSTIT_GIT_URL) POSTIT_GIT_TAG=$(POSTIT_GIT_TAG) \
+	POSTIT_GIT_URL=$(POSTIT_GIT_URL) POSTIT_GIT_TAG=$(POSTIT_GIT_TAG) POSTIT_RUNTIME=$(POSTIT_RUNTIME) \
 	    dpkg-buildpackage -us -uc -b
 	mv ../postit_*$(POSTIT_GIT_TAG)*.deb $(POSTIT_OUT_DIR)/ 2>/dev/null || \
 	    mv ../postit_*.deb $(POSTIT_OUT_DIR)/ || true

README.md

@@ -61,9 +61,22 @@ sudo apt -f install     # résout les dépendances manquantes
 Pour installer sur **arm64** :
 
 ```bash
-make clean && make deb   # sur une machine arm64, le .deb sera _arm64.deb
+# Sur une machine arm64 :
+make clean && make deb   # le .deb sera _arm64.deb
 ```
 
+Le `debian/rules` publie avec `--runtime linux-x64` pour
+amd64. Pour produire un .deb arm64, override :
+
+```bash
+make deb POSTIT_RUNTIME=linux-arm64
+```
+
+(le paramètre est exposé par `debian/rules` ; bump à ajouter si
+besoin). À noter : **un seul runtime par build** — le `--runtime`
+exclut les autres plateformes, donc le .deb est portable mais
+mono-architecture.
+
 ## Vérification
 
 Après installation :

debian/rules

@@ -21,6 +21,10 @@
 
 POSTIT_GIT_URL ?= https://github.com/pazof/yavsc.git
 POSTIT_GIT_TAG ?= 1.0.0
+# Runtime identifier passed to `dotnet publish --runtime`. Override
+# at build time for non-amd64 targets:
+#   make deb POSTIT_RUNTIME=linux-arm64
+POSTIT_RUNTIME ?= linux-x64
 POSTIT_DESKTOP_PROJECT := src/PostIt/PostIt.Desktop/PostIt.Desktop.csproj
 POSTIT_BUILD_DIR := $(CURDIR)/build
 POSTIT_SRC_DIR := $(POSTIT_BUILD_DIR)/yavsc-src
@@ -56,7 +60,7 @@ override_dh_auto_build:
 	dotnet publish $(POSTIT_SRC_DIR)/$(POSTIT_DESKTOP_PROJECT) \
 	    -c Release \
 	    --self-contained false \
-	    --runtime linux-x64 \
+	    --runtime $(POSTIT_RUNTIME) \
 	    -p:PublishReadyToRun=false \
 	    -o $(POSTIT_BUILD_DIR)/postit-app