fix(setup): locate fbuild binary via cargo --message-format=json

[zackees]

Summary

Drives cargo with --message-format=json-render-diagnostics and pulls the binary path out of the structured artifact stream — the same mechanism cargo install uses. Works regardless of how cargo lays out its target dir.

Relation to #461

PR #461 already shipped a fix for the same bug (filesystem probe of target/release/ + every target/<triple>/release/ subdir, max-mtime tiebreak). This PR is rebased on top of that merge and replaces the filesystem probe with the JSON-stream approach. Both fix the original "binary not at target\release\fbuild.exe" failure. This PR is materially more robust against:

• Custom CARGO_TARGET_DIR — env-driven override, doesn't sit anywhere under the repo's target/ tree at all.
• Custom profiles (--profile dist, etc.) — output dir becomes target/<triple>/<profile>/, which the mtime-tiebreak filesystem probe in fix(setup.py): locate cargo binary under target/<host-triple>/release/ #461 can't see.
• Cargo's own conventions — if cargo ever rearranges its output layout (it has, twice in the last decade), the JSON stream still reports the right path.

The filesystem probe survives as a fallback for the cached-Fresh edge case where cargo doesn't emit compiler-artifact messages on stdout. So this PR is strictly a superset of #461's behaviour.

What changed

• Runs cargo build --release -p fbuild-cli --message-format=json-render-diagnostics.
• Captures stdout (JSON stream); stderr passes through so soldr's session summary stays visible.
• Walks the artifact lines, picks the most recent compiler-artifact with target.name == "fbuild" and a non-null executable field.
• Falls back to probing canonical layouts when the JSON stream produces nothing.
• Error message when both fail lists every candidate path and points users at the issue tracker with the verbose-build command attached.

Test plan

• Verified locally on Windows: uv pip install succeeds where the original target/release/<bin> check failed.
• Binary lands at ci/bin/fbuild.exe as expected by the wheel packaging step.
• fbuild --version from the newly-installed venv prints fbuild 2.2.19.
• CI on this branch will exercise the cross-platform paths.

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 7002546c-ce6a-4eaa-a4a0-089ad7413623

📥 Commits

Reviewing files that changed from the base of the PR and between 3464d1c and 3368f01.

📒 Files selected for processing (1)

• setup.py

📝 Walkthrough

Walkthrough

The PR strengthens setup.py's binary discovery logic by parsing Cargo's JSON diagnostic output to locate the compiled fbuild executable, handling per-host and per-layout target layouts. A filesystem search fallback is added if no JSON artifact is found, and the discovered binary is staged into ci/bin/.

Changes

Fbuild Binary Discovery

Layer / File(s)	Summary
Documentation and imports setup.py	Module docstring expanded to document Cargo's per-host/per-layout binary locations and JSON-based discovery strategy; json and Optional imports added and TARGET_DIR removed.
Binary discovery implementation setup.py	_find_fbuild_executable_from_json() parses Cargo JSON output for the compiler-artifact entry; _find_fbuild_executable_by_search() searches target/ directories as a fallback; _build_fbuild_cli() runs soldr cargo build, orchestrates discovery, and stages the found binary.
Build process integration setup.py	BuildWithCargo.run() now invokes _build_fbuild_cli() to build, discover, and stage the executable into ci/bin/ and fails with a clear error if the binary is not found.

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

Poem

I nibble JSON lines in the night,
Sniffing artifacts till paths are right,
No brittle targets block my way,
I fetch the binary, tuck it away,
A rabbit's hop — build saved today. 🐇✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and specifically describes the main change: improving the fbuild binary location discovery in setup.py by using cargo's JSON message format instead of assuming a fixed target directory layout.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/setup-py-target-triple

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