fix: avoid mapfile in upload-release-to-gcs.sh

[ValentaTomas]

`mapfile` is bash 4+; macOS ships bash 3.2 at `/bin/bash`, so running the script locally errored with `mapfile: command not found`. Swap for a portable `while IFS= read -r line; do ASSETS+=("$line"); done < <(…)` loop.

Verified on `GNU bash 3.2.57` (macOS):

```
$ ./scripts/upload-release-to-gcs.sh --hash c1a568c --dry-run --bucket e2b-staging-fc-kernels
Release: 2026.05.11 (commit c1a568c)
Target: gs://e2b-staging-fc-kernels
Mode: dry-run
WOULD vmlinux-6.1.102-amd64.bin -> gs://e2b-staging-fc-kernels/vmlinux-6.1.102-c1a568c/amd64/vmlinux.bin
... (3 more)
Dry run complete. Already in GCS: 0.
```

[cursor]

PR Summary

Low Risk
Low risk doc and scripting changes; main impact is altering the GCS object path convention and the way release assets are enumerated.

Overview
Fixes upload-release-to-gcs.sh failing on macOS bash 3.x by replacing mapfile with a portable read loop, and changes the GCS destination path to use vmlinux-<version>_<short_hash> (underscore) instead of vmlinux-<version>-<short_hash>. Updates README/docs to match the new upload path.

^{Reviewed by Cursor Bugbot for commit 1738214. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

Bugbot Autofix prepared a fix for the issue found in the latest run.

• ✅ Fixed: Last line silently dropped if input lacks trailing newline
  • Added '|| [[ -n "$line" ]]' to the while loop condition to ensure the last line is processed even without a trailing newline.

Or push these changes by commenting:

@cursor push dd8718b61c

Preview (dd8718b61c)

diff --git a/scripts/upload-release-to-gcs.sh b/scripts/upload-release-to-gcs.sh
--- a/scripts/upload-release-to-gcs.sh
+++ b/scripts/upload-release-to-gcs.sh
@@ -74,7 +74,7 @@
 $DRY_RUN && echo "Mode:    dry-run"
 
 ASSETS=()
-while IFS= read -r line; do
+while IFS= read -r line || [[ -n "$line" ]]; do
   [[ -n "$line" ]] && ASSETS+=("$line")
 done < <(gh release view "$RELEASE_TAG" --repo "$REPO" --json assets \
   --jq '.assets[] | select(.name | test("^vmlinux-.*\\.bin$")) | .name')

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^{Reviewed by Cursor Bugbot for commit ce2d4f3. Configure here.}