build-one: add firmware_ref input for cross-repo bisect (#94)

When investigating a regression that lives in OpenIPC/firmware (or its
referenced packages like hisilicon-opensdk) but only manifests in a
builder build, you previously had to either roll firmware master back
or fork builder.sh manually. Adds a clean way:

  gh workflow run build-one.yml -f platform=hi3516ev200_fpv \
                                -f firmware_ref=<sha-or-tag-or-branch>

The workflow exports OPENIPC_FW_REV as env, which builder.sh now
honours: when set, it does a full clone (not --depth=1) and checks
out the requested ref before invoking the firmware build. When unset,
behaviour is unchanged (shallow clone of master HEAD).

Immediate motivation: bisect the +180 KB rootfs growth in
hi3516ev200_fpv between 2026-05-08 (5016 KB) and 2026-05-24 (5196 KB,
76 KB over partition limit). Per kaeru
'hi3516ev200-ev300-fpv-rootfs-size-overflow-2026-05-24'. Suspect is
recent hisilicon-opensdk bumps that brought in source-built sensor
drivers for ev200; bisect across firmware commits varying
HISILICON_OPENSDK_VERSION will pinpoint which bump pushed us past
the 5120 KB limit.

Generally useful long after this specific incident:
- Reproducing user-reported regressions at a specific firmware tag.
- Locking builds to a known-good firmware revision when builder is
  ahead of firmware.
- Future cross-repo bisects of any kind.

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

.github/workflows/build-one.yml

@@ -6,7 +6,10 @@ on:
         description: 'Platform to build (e.g. hi3518ev200_lite_switcam-hs303-v2)'
         required: true
       commit:
-        description: 'Commit SHA to build (optional; defaults to current branch HEAD). Use this from `git bisect run` or for one-off historic rebuilds.'
+        description: 'Builder commit SHA to build (optional; defaults to current branch HEAD). Use this from `git bisect run` or for one-off historic rebuilds.'
+        required: false
+      firmware_ref:
+        description: 'OpenIPC/firmware ref (SHA/tag/branch) to clone — for cross-repo bisect of size/regression issues. Defaults to firmware HEAD.'
         required: false
 
 jobs:
@@ -60,6 +63,7 @@ jobs:
           BUILD_ID:       ${{ needs.resolve.outputs.tag_name }}
           BUILD_SHA:      ${{ needs.resolve.outputs.ref }}
           BUILD_PLATFORM: ${{ inputs.platform }}
+          OPENIPC_FW_REV: ${{ inputs.firmware_ref }}
         run: |
           export GIT_HASH=$(git rev-parse --short ${GITHUB_SHA})
           export GIT_BRANCH=${GITHUB_REF_NAME}