Force IPv4 in the build VM, where apt was losing 31s per large fetch

Every apt download over roughly 15 MB inside the build VM took exactly 31
seconds regardless of its size, while a 14.1 MB one took 1 second at 23 MB/s.
A fixed cost that ignores size is a timeout, not a bandwidth limit: QEMU's
user-mode network advertises IPv6 that does not work, so apt's parallel
connections black-holed on it and only fell back to IPv4 when the 30-second
timeout expired.

Patch the build VM's cloud-init seed from bootcmd, which runs in the
init-local stage before SSH is up, so the setting covers upstream's apt calls
as well as ours. The same 28.5 MB fetch went from 31s (914 kB/s) to 3s
(9152 kB/s).

Also fix APT_PROXY, which never worked as documented. A cache cannot see
inside a CONNECT tunnel, so the repositories have to be rewritten to plain
http first, and Debian 13 keeps the real mirror URLs in /etc/apt/mirrors/*.list
behind the mirror+file: method, which rewriting sources.list alone misses.

The README now carries the measurements rather than estimates. The headline is
that the cache barely matters: with it fully warm it saves about three seconds
of a 4m40s build, so a local Debian mirror (138 GB for trixie amd64) would buy
nothing here.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Makefile

@@ -162,6 +162,16 @@ image: checkout $(CUSTOMIZE)
 	sed -i -E 's|^([[:space:]]*disk_size[[:space:]]*=[[:space:]]*).*|\1"$(DISK_SIZE)"|' $(TPL)/debian-cloudimg.pkr.hcl
 	sed -i -E 's|^([[:space:]]*cpus[[:space:]]*=[[:space:]]*).*|\1$(BUILD_CPUS)|' $(TPL)/debian-cloudimg.pkr.hcl
 	sed -i -E 's|^([[:space:]]*memory[[:space:]]*=[[:space:]]*).*|\1$(BUILD_MEM)|' $(TPL)/debian-cloudimg.pkr.hcl
+	@echo "==> Build VM apt ayari yamalaniyor (IPv4 zorlama + timeout)"
+	# Every apt fetch over roughly 15 MB inside the build VM stalls for exactly
+	# 31 seconds regardless of its size, which is a connection timeout rather
+	# than a bandwidth limit: QEMU's user-mode network offers IPv6 that does not
+	# actually work, and apt's parallel connections black-hole on it before
+	# falling back to IPv4. bootcmd runs in cloud-init's init-local stage,
+	# before SSH is up, so the setting is in place for upstream's apt calls too.
+	sed -i "/^bootcmd:/a\\  - echo 'Acquire::ForceIPv4 \"true\"; Acquire::http::Timeout \"20\"; Acquire::Retries \"3\";' > /etc/apt/apt.conf.d/99-build-speed" $(TPL)/user-data-cloudimg
+	@sed -n '/^bootcmd:/,+1p' $(TPL)/user-data-cloudimg
+
 ifeq ($(strip $(DEBIAN_IMAGE_CHANNEL)),stable)
 	@echo "==> Kararli Debian cloud image kullanilacak (packer onbellegi isabet eder)"
 	sed -i -E 's|/daily/latest/|/latest/|g; s|-daily\.qcow2|.qcow2|g' $(TPL)/debian-cloudimg.pkr.hcl