fix(pi-fff): isomorphic bun/node SDK lazy-load

[gustav-fff]

Closes #668

Root cause

packages/pi-fff/src/index.ts:21 had a top-level import { FileFinder } from "@ff-labs/fff-node". @ff-labs/fff-node pulls ffi-rs and the optional per-platform native binary packages (@ff-labs/fff-bin-*) into its module graph. oh-my-pi statically resolves every extension import to validate the extension; the optional native package for a foreign platform is not on disk, so validation fails even though the runtime install would be fine.

Reporter also flagged that pi runs under both bun and node — but pi-fff only ever loaded the node SDK, so bun hosts paid the ffi-rs cost instead of using @ff-labs/fff-bun which shares the same FileFinderApi.

Fix

• Drop the static FileFinder value import; keep @ff-labs/fff-node for type-only imports (erased at compile time, invisible to the static validator).
• New detectRuntime() checks globalThis.Bun and process.versions.bun.
• New loadSdk() does a variable-name await import(pkg) for the runtime-appropriate SDK, with a fallback to the other one. Variable-name import defeats static resolvers.
• Result cached in sdkPromise so a single load survives concurrent tool calls.
• ensureFinder() now await loadSdk() before FileFinder.create(...); the existing in-flight-promise guard from pi-fff: parallel ffgrep calls can hang indefinitely due to unsynchronized ensureFinder singleton #403 still serializes creation.
• package.json: add @ff-labs/fff-bun as an optional peer dependency so bun-hosted pi pulls it in without forcing node-hosted pi to install it.

Both SDKs implement the identical FileFinderApi (packages/shared/fff-api.ts:528), so the call sites don't change.

Steps to reproduce

git checkout main
cd packages/pi-fff
bun install
# The static import chain is visible to any resolver that walks it:
node -e "console.log(require.resolve('@ff-labs/fff-node'))"
# oh-my-pi's static validator follows @ff-labs/fff-node -> ffi-rs -> optional native binary,
# which is missing for platforms other than the host and aborts validation.

Expected: extension loads under oh-my-pi.
Actual: validator errors on the unresolved optional native package.

How verified

• cd packages/pi-fff && bun run typecheck — clean.
• bun test test/ — 18/18 pass (added mock for @ff-labs/fff-bun alongside @ff-labs/fff-node since bun-hosted tests now prefer the bun SDK).
• bun run format — no changes.

Not verified end-to-end against a real oh-my-pi install — that would need the pi validator in the loop. @dmtrKovalenko please smoke-test under omp before releasing.

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[gustav-fff]

[triage-bot] CI note: Spell Check with Typos fails on crates/fff-core/src/bigram_query.rs:202 (simpliest → simplest). Pre-existing on main — same failure on the latest main run: https://github.com/dmtrKovalenko/fff/actions/runs/29052646755. Unrelated to this diff.

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