fix(ci): shard single-arch backend matrix under GitHub's 256-job limit

[localai-bot]

Problem

The v4.6.1 backend build (run 28786533892) concluded failure while every job in the run was green — and the backend-jobs-singlearch / backend-merge-jobs-singlearch groups sat forever at "Waiting for pending jobs".

Root cause: GitHub Actions refuses to instantiate a matrix that would generate more than 256 jobs, and it does so silently — the caller job hangs at "Waiting for pending jobs" and the run is marked failure with no failing job to point at. The single-arch build matrix had grown to 268 entries (> 256), so those jobs were never created. That's why they don't appear in the jobs API or check-runs, yet the release build "failed".

generate-matrix          1
backend-jobs-multiarch   111   ✅
backend-jobs-darwin      106   ✅
backend-merge-multiarch  57    ✅
backend-jobs-singlearch  0     ❌ never instantiated (268 > 256)
backend-merge-singlearch 0     ❌

Fix

The single-arch list is the one that grows unbounded as backends are added, so shard it across a fixed number of matrix jobs:

• SINGLEARCH_SHARDS = 4 in changed-backends.js — the 268 entries split into 4 balanced shards of ~67 (headroom to ~1020 backends before the next bump).
• Each merge shard needs: only its matching build shard, preserving the "merge waits only on its own build" property (PR ci: split backend-jobs into single-arch and multi-arch matrices #9746) that keeps slow CUDA/ROCm single-arch builds from gating multi-arch manifest assembly.
• The script throws loudly if any shard ever reaches the 256 limit, naming exactly what to bump — instead of letting GitHub drop the overflow silently again.
• backend.yml and backend_pr.yml gain the four build + four merge shard jobs. Multi-arch and darwin groups are untouched.

Validation

• Ran changed-backends.js against the full matrix: 268 → shards [67, 67, 67, 67], total preserved, no aggregate output leaked, multi-arch (111) / darwin (53) unchanged.
• Unit-tested chunkEqually edge cases (0–7 entries, 268, 1020, 1024): balanced, sums preserved, guard trips exactly at 1024 (256/shard).
• actionlint passes clean on both workflows; both parse and each merge shard correctly needs: its build shard; fail-fast semantics preserved (false in backend.yml, true in backend_pr.yml).

🤖 Generated with Claude Code