feat(vanilla): add bundled build strategy to reduce MultiCompiler fan-out - #54

What?

Release v2.5.0: add an opt-in strategy: 'bundled' option to getVanillaConfig, bundled entry-key modes for consumer-facing output paths, and runWebpackConfigs for concurrency-limited builds.

Why?

The legacy per-file vanilla strategy creates one webpack config per source file per variant (4 × file count). webpack-cli runs these via MultiCompiler in parallel, which drove high peak memory and slow builds in Polylang Pro before glob fixes — and remains a bottleneck for any consumer with many vanilla assets.

Polylang-family plugins enqueue flat build URLs (js/build/settings.min.js). Bundled builds default to the same flat layout as per-file (entryKey: 'basename'), so consumers need not pass entryKey or maintain local webpack hacks.

How?

• strategy: 'bundled' (default remains 'per-file'): emits up to 4 configs — JS min/unmin + CSS min/unmin — using multi-entry webpack configs. Invalid strategy values throw (no silent fallback to per-file).
• entryKey (bundled only): 'basename' (default) for flat js/build/{name} paths, or 'relative' for nested outputs mirroring source paths. Duplicate basenames in one bundled config throw at config generation time.
• createBundledJsConfig / createBundledCssConfig: collision-safe entry maps; splitChunks / runtimeChunk disabled so each entry stays one file; bundled CSS scopes CleanWebpackPlugin to the output directory.
• CSS intermediates: min/unmin use distinct .min.work / .unmin.work files so bundled (and per-file) CSS min + unmin can run in parallel without racing on the same paths. Per-file CSS cleanup is scoped to the output directory instead of process.cwd().
• Glob dedupe: stable deduplication when multiple patterns match the same file.
• runWebpackConfigs(configs, { concurrency }): TypeError for non-array configs or invalid concurrency; fail-fast on first compilation error; Infinity concurrency still supported.

Versioning

• Bump @wpsyntex/polylang-build-scripts to 2.5.0 (minor): new public APIs plus CSS intermediate filename and clean-scope changes that apply to all vanilla strategies (not only bundled).

Test plan

• npm test (77 tests)
• npm run lint
• Integration: nested bundled outputs (entryKey: 'relative'), default flat JS/CSS, parallel bundled CSS min/unmin, no extra chunks with shared local imports
• Consumer smoke test: Polylang Pro with strategy: 'bundled' only (~6 total configs instead of ~54)