docs: add backlog task for cargo-binstall tarpaulin CI debug

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pythoninthegrass

backlog/tasks/task-300 - Debug-cargo-binstall-PATH-issue-for-tarpaulin-in-CI.md

@@ -0,0 +1,76 @@
+---
+id: TASK-300
+title: Debug cargo-binstall PATH issue for tarpaulin in CI
+status: To Do
+assignee: []
+created_date: '2026-03-19 04:00'
+updated_date: '2026-03-19 04:02'
+labels:
+  - ci
+  - bug
+dependencies: []
+references:
+  - >-
+    https://github.com/pythoninthegrass/mt/actions/runs/23278515091/job/67686711086
+  - .github/workflows/ci.yml
+priority: high
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+## Problem
+
+After switching to `cargo-binstall` for installing `cargo-tarpaulin` in CI (commit `cf09f50`), the "Build & Rust Tests" job fails with:
+
+```
+error: no such command: `tarpaulin`
+```
+
+The `cargo binstall --no-confirm cargo-tarpaulin` step succeeds — it reports `cargo-tarpaulin v0.35.2 is already installed` (from cache). However, the subsequent `cargo tarpaulin` command cannot find the binary.
+
+## Root Cause (Suspected)
+
+PATH / CARGO_HOME mismatch. The `cargo-binstall` action installs the binary to one location, but `cargo` looks for subcommands in a different directory. Key observations from the logs:
+
+- `CARGO_HOME=/Users/lance/.cargo` (set by binstall action)
+- PATH includes `/Users/lance/.cargo/bin` (set by Tauri build environment step)
+- binstall reports the binary is "already installed" — likely cached from a previous `cargo install` run and sitting in a different bin dir than where binstall would place it
+- The self-hosted runner may have stale cached binaries in a location no longer on PATH
+
+## Relevant CI Config
+
+- Workflow: `.github/workflows/ci.yml`
+- Steps: "Run cargo-bins/cargo-binstall@main" → "Install cargo-tarpaulin" → "Run Rust tests with coverage"
+- Runner: self-hosted macOS (ARM64)
+
+## Failed Run
+
+https://github.com/pythoninthegrass/mt/actions/runs/23278515091/job/67686711086
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [ ] #1 cargo-tarpaulin installs and runs successfully via cargo-binstall in CI
+- [ ] #2 Coverage report is generated and uploaded as artifact
+- [ ] #3 CI pipeline passes end-to-end on self-hosted macOS runner
+<!-- AC:END -->
+
+## Implementation Notes
+
+<!-- SECTION:NOTES:BEGIN -->
+## Investigation Findings
+
+- **Build & Rust Tests** ran on runner `mini` (Mac Mini), NOT `studio` (Mac Studio)
+- **Playwright E2E Tests** ran on runner `studio`
+- The Mac Mini may not have cargo-binstall installed at the same paths, or may have a stale cached tarpaulin binary in a location not on PATH
+- binstall reported "cargo-tarpaulin v0.35.2 is already installed" — this means it found a metadata record but the actual binary may not be where cargo looks for it
+- Key question: does the Mac Mini have the same CARGO_HOME / PATH setup as the Studio? The previous `cargo install` approach may have placed tarpaulin in a different bin directory than where binstall expects it
+
+## Debug Steps
+
+1. SSH into Mac Mini and check `which cargo-tarpaulin`, `ls ~/.cargo/bin/cargo-tarpaulin`
+2. Compare CARGO_HOME and PATH between Mini and Studio runners
+3. Consider adding `--force` flag to binstall to ensure fresh install to correct location
+4. Add `which cargo-tarpaulin` and `echo $PATH` debug steps to CI workflow
+<!-- SECTION:NOTES:END -->