build: stabilize zccache compile cwd

[zackees]

Summary

• run zccache-wrapped compiles from the workspace root inferred from .fbuild outputs
• keep non-.fbuild output layouts on the previous inherited-CWD path
• add an integration test that builds identical workspaces at two absolute paths and asserts the second compile is a zccache hit

zccache audit

Upstream zackees/zccache normalizes C/C++ source, include, and header paths relative to the request key_root; the daemon registers that key_root from the compile request CWD. Output paths are not part of the per-TU key. That means fbuild should invoke zccache from the project workspace root, not from a runner-specific process CWD, for cross-runner/workspace-stable keys.

Relevant upstream references:

• https://github.com/zackees/zccache/blob/b0436da318f109f1180ee78b8392d137523f39e2/crates/zccache-depgraph/src/context.rs#L116-L131
• https://github.com/zackees/zccache/blob/b0436da318f109f1180ee78b8392d137523f39e2/crates/zccache-depgraph/src/context.rs#L195-L210
• https://github.com/zackees/zccache/blob/b0436da318f109f1180ee78b8392d137523f39e2/crates/zccache-daemon/src/server.rs#L2783-L2787
• https://github.com/zackees/zccache/blob/b0436da318f109f1180ee78b8392d137523f39e2/crates/zccache-daemon/tests/stress_test.rs#L708-L745
• https://github.com/zackees/zccache/blob/b0436da318f109f1180ee78b8392d137523f39e2/crates/zccache-daemon/tests/stress_test.rs#L757-L838

Fixes #150

Testing

• uv run soldr cargo fmt --all -- --check
• uv run soldr cargo test -p fbuild-build --test zccache_hit_across_workspace_rename
• uv run soldr cargo test -p fbuild-build zccache::tests::

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📥 Commits

Reviewing files that changed from the base of the PR and between 888ec98 and 5fe8866.

📒 Files selected for processing (3)

• crates/fbuild-build/src/compiler.rs
• crates/fbuild-build/src/zccache.rs
• crates/fbuild-build/tests/zccache_hit_across_workspace_rename.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/zccache-cross-workspace-hit

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