install.sh: fix four bugs found during live testing

Free-space start alignment
  parted reports free space starting at 0,02 MiB (before the GPT
  alignment boundary). The collect_free_space awk now rounds the start
  up to the next whole MiB (ceiling) and enforces a minimum of 1 MiB,
  then recomputes the usable size from the adjusted start. This prevents
  parted from being asked to create a partition at 0 MiB, which it
  cannot do.

Locale-independent partition creation
  The previous `printf 'Yes\n' | parted mkpart` relied on parted
  accepting an English answer to its alignment-confirmation prompt.
  On a German-locale system parted asks "Ist dies noch akzeptabel?"
  and ignores "Yes", causing mkpart to fail. Replaced with `parted -s`
  (script/non-interactive mode), consistent with every other parted
  call in the script.

Correct new-partition detection on disks with gaps
  The old heuristic took the highest partition number after partprobe.
  On a disk where existing partitions are numbered 2/3/4, a new
  partition in the gap before them receives number 1 — making the
  old heuristic point at partition 4 (the existing btrfs volume) and
  subsequently run mkfs.btrfs on it. The new awk matches by start
  position (OEMDRV_START ± 1 MiB) instead, which is unambiguous
  regardless of how numbers are assigned.

Infinite loop on EOF stdin
  When the selection while-loop's `read` hits EOF (e.g. stdin exhausted
  after sudo consumed a piped password), it returns exit code 1 with an
  empty INPUT, which falls through to "Invalid input." and spins
  forever. Added `|| { echo; echo "Aborted."; exit 0; }` to all three
  read calls in the loop.

install.md: drop stale install_from_repo.sh reference from title;
clarify that REPO_URL/REPO_BRANCH overrides are optional.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

install.md

@@ -1,4 +1,4 @@
-# OEMDRV Bootstrap — install.sh + install_from_repo.sh
+# OEMDRV Bootstrap — install.sh
 
 the script `./system_setup/install.sh` prepares a target machine for automated Fedora deployment. It shrinks an existing partition to carve out a dedicated **OEMDRV** partition, which Anaconda/Kickstart will detect automatically during installation.
 
@@ -37,7 +37,7 @@ curl -fsSL ${REPO_URL%.git}/raw/branch/${REPO_BRANCH:-main}/system_setup/install
 sudo -E bash /tmp/install.sh
 ```
 
-That way, install.sh should know what to pull.
+Both are optional. That way, install.sh should know what to pull.
 
 ## After the script completes