fix(teensy): derive map_path from elf_path (unbreak main CI)

[zackees]

Summary

build_link_args (added by #313) inherited a hold-over from #305 that referenced output_dir.join(\"firmware.map\") — but output_dir isn't a parameter of build_link_args; it takes elf_path: &Path. The workspace stopped compiling on every PR built against main since #313 landed:

error[E0425]: cannot find value \`output_dir\` in this scope
  --> crates/fbuild-build/src/teensy/teensy_linker.rs:93:24

Every open PR (#306, #307, #315, …) is red because of this. Hotfix first so the other PRs can get green CI.

Fix

Derive the map path from `elf_path.with_extension("map")`, which is equivalent to `output_dir.join("firmware.map")` since `elf_path = output_dir.join("firmware.elf")` at the only call site. One-line change.

Adds `test_teensy_link_command_emits_linker_map_next_to_elf` to catch this if anyone factors the path source again.

Test plan

• `soldr cargo check -p fbuild-build` — clean
• `soldr cargo test -p fbuild-build --lib teensy::teensy_linker` — 6/6 pass (5 existing + 1 new)
• `soldr cargo clippy -p fbuild-build --all-targets -- -D warnings` — clean

🤖 Generated with Claude Code

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ℹ️ Review info

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Run ID: 04a7c0f9-fd9c-45ac-8e13-a7e6a7f53c2c

📥 Commits

Reviewing files that changed from the base of the PR and between 3cf0eed and 2d93746.

📒 Files selected for processing (1)

• crates/fbuild-build/src/teensy/teensy_linker.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/teensy-linker-output-dir-scope

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