chore: Update fff to 0.9.4

[dmtrKovalenko]

Issue for this PR

Closes #31584

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Bumps @ff-labs/fff-bun from 0.9.3 to 0.9.4 and wires up the new embedded-lib resolution it introduces.

0.9.4 embeds its native library into bun build --compile output via a type: "file" import plus a build-time FFF_LIBC constant (gnu/musl), since libc cannot be detected at build time on Linux. When the define is missing, the embedded resolver falls back to gnu, so musl Linux binaries would embed the wrong .so. This PR:

• Updates the dependency and lockfile to 0.9.4.
• Passes FFF_LIBC via bun build --define for Linux targets in both binary build scripts that bundle fff: packages/opencode/script/build.ts and packages/cli/script/build.ts, mirroring the existing OPENCODE_LIBC/OPENTUI_LIBC handling (defaults to gnu, musl for the abi-specific builds).
• Adds a declare global for FFF_LIBC next to the fff wrapper so type checking of the package's embedded.ts resolves the constant when consumed from source.

How did you verify your code works?

• bun typecheck passes for both opencode and cli (previously failed with Cannot find name 'FFF_LIBC').
• Ran fff/filesystem/tool tests in packages/opencode and packages/core (glob, grep, read, filesystem search) — all pass.
• Built standalone binaries for both opencode and cli (build.ts --single) and confirmed the fff native lib is embedded (/$bunfs/root/libfff_c-*.dylib) and loads at runtime: opencode debug file search returns correct results from the compiled binary.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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[github-actions]

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