Use bun as the kit package manager and script runner

[mishig25]

What

Switches the kit toolchain from npm to bun for installing dependencies and running package scripts, across local commands (doc-builder build --html, doc-builder preview) and all CI workflows.

What bun is (and isn't) here: bun replaces npm (installs, ~20s → <1s warm / ~5s cold) and the script runner. It does not replace ESLint/Prettier — bun ships no linter/formatter (that would be Biome, which still has partial Svelte support and no tailwind class sorting, so it's not a good trade here). And Vite itself still runs on Node.js (≥ 20): bun is the installer/runner, node stays the battle-tested runtime for the build — the python toolchain check now verifies both.

Changes

• kit/bun.lock replaces package-lock.json — generated against the public registry, resolving the same dependency versions (verified, incl. the @huggingface/inference@4.13.15 pin). Bonus: bun.lock stores no registry URLs, so it's structurally immune to the internal-mirror-baked-URL trap that once broke CI with package-lock.json.
• build.py/preview.py: bun install --frozen-lockfile + bun run build|dev; check_toolchain_is_available() checks bun ≥ 1.2 and node ≥ 20 up front.
• CI: setup-bun added to the SvelteKit lint workflow, the shared build_main_documentation/build_pr_documentation workflows, and the accelerate integration workflow. setup-node is kept in the build workflows on purpose: they can run in custom containers where node isn't preinstalled.
• bump-huggingface-js-packages workflow and README dev instructions updated (bun install, bun run dev, bun run update-inference-providers).

Verification

• Accelerate e2e: output identical to the npm-built output on all 48 pages. One finding along the way: the hydration node_ids pointers differ across any two builds (npm↔npm or bun↔bun alike) because SvelteKit enumerates route nodes with unsorted readdirSync — each build is internally consistent, and everything else is byte-identical once that's normalized.
• Browser tests on the bun-built output: hydration, SPA navigation, shorthand-URL preservation, HfOptions tabs, docstring tooltips — all pass.
• doc-builder preview smoke-tested: bun install + vite dev server + page serving work end-to-end.
• ruff and pytest clean.

🤖 Generated with Claude Code

[mishig25]

Closing for now: checking the transformers workflow surfaced two blockers for a safe rollout —

1. custom_container images (e.g. huggingface/transformers-doc-builder, FROM python:3.10) have no Node; it was provided by the setup-node step this PR removes, so container-based doc builds would lose Vite's runtime.
2. Most consumer repos (transformers, accelerate, diffusers, datasets, peft, trl) pin the shared build workflows to old commit SHAs while those workflows checkout doc-builder code at main — merging this would immediately break their doc builds ("bun was not found") until every repo bumps its pin.

The branch is kept. A safe re-roll would need: setup-node restored in the shared workflows, an npm fallback in the python commands with package-lock.json kept alongside bun.lock during the transition, and ideally the shared workflows checking out doc-builder at their own ref so future toolchain changes are atomic for pinned consumers.