fix(opencode): bundle fff native library

[thdxr]

Summary

• select the Bun filesystem implementation while retaining Node package conditions
• patch @ff-labs/fff-bun to expose its platform library through statically analyzable relative require(...) templates
• embed the FFF native library in compiled standalone binaries

Verification

• bun typecheck from packages/opencode
• bun run script/build.ts --single --skip-install --skip-embed-web-ui
• started the compiled binary with serve --print-logs in tmux
• requested GET /find/file?query=build.ts and received HTTP 200 with FFF results
• confirmed no adjacent libfff_c.so is required

Notes

git diff --check reports the two blank unified-diff context lines inside the Bun dependency patch. Those leading spaces are required by Bun's patch parser; removing them makes bun install reject the patch.

[greptile-apps]

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[rexdotsh]

ran into the same issue when building beta, was just about to send a PR..
take a look at this may be while you're testing out fff capabilities? #31556