feat(acceptance): #205 AC#6 teensy41 cold resolver <= 200 ms gate

[zackees]

Closes the last unmet acceptance criterion of #205. With this merge, all six ACs are CI-enforced and #205 can close.

AC#6 verbatim from the #205 issue body

6. Cold run of library selection on teensy41 project <= 200 ms on a typical CI runner.

Summary

• New integration test teensy41_cold_library_selection_meets_205_ac6 in crates/fbuild-build/tests/teensy41_acceptance.rs materializes a real Teensyduino install via fbuild_packages::library::TeensyCores::ensure_installed, lists its 92 bundled framework libraries with get_framework_libraries, and times a single uncached fbuild_library_select::resolve(seeds, search_paths, libraries) call against a teensy41 Blink fixture.
• Asserts elapsed_ms <= 200.0 with a clear panic message; emits the timing on stdout so the CI log records it.
• Wired into .github/workflows/acceptance-205.yml as a fourth matrix entry; the same workflow already covers AC#1 (teensyLC), AC#2 (teensy30 AnalogOutput), and AC#4 (stm32f103c8 SPI).
• Uses the inline-tempdir pattern from stm32_acceptance.rs / teensy30_acceptance.rs — no committed fixture, no scratch artifacts in the repo.

TDD walkthrough

RED — assertion mechanism proven

Temporarily tightened the threshold to 0.001 ms and ran:

uv run soldr cargo test -p fbuild-build --release --test teensy41_acceptance \
    -- --ignored teensy41_cold_library_selection_meets_205_ac6 --nocapture

Output:

AC#6 teensy41 framework installed at C:\Users\niteris\.fbuild\prod\cache\platforms\dl-framework-arduinoteensy-1.160.0\1b0675c97b0fd8d7\1.160.0
AC#6 teensy41 framework libraries discovered: 92
AC#6 cold resolve: 11.09 ms (budget 200.00 ms), selected 0 library(ies)
thread 'teensy41_cold_library_selection_meets_205_ac6' panicked at crates\fbuild-build\tests\teensy41_acceptance.rs:126:5:
AC#6: cold resolve must finish in <= 200 ms; got 11.09 ms
test teensy41_cold_library_selection_meets_205_ac6 ... FAILED

Panic format matches the spec; the assertion mechanism works.

GREEN — restored threshold passes

Restored the threshold to 200.0 and reran the same command:

AC#6 teensy41 framework installed at C:\Users\niteris\.fbuild\prod\cache\platforms\dl-framework-arduinoteensy-1.160.0\1b0675c97b0fd8d7\1.160.0
AC#6 teensy41 framework libraries discovered: 92
AC#6 cold resolve: 11.36 ms (budget 200.00 ms), selected 0 library(ies)
test teensy41_cold_library_selection_meets_205_ac6 ... ok

Measured cold time: 11.36 ms — ~17.6x headroom under the 200 ms budget. Plenty of margin for slower CI runners and growth in the Teensyduino library set.

Files changed

File	LOC	Purpose
crates/fbuild-build/tests/teensy41_acceptance.rs	+130	New integration test for AC#6
.github/workflows/acceptance-205.yml	+4	Add teensy41 cold resolver matrix entry + path filter

#205 AC status after this PR

AC	Contract	Gate test	Status
AC#1	teensyLC .bss <= 3 KB + no FNET/Snooze/RadioHead/mbedtls	teensylc_blink_meets_205_acceptance_criteria	CI-enforced
AC#2	teensy30 AnalogOutput .dmabuffers <= 1 KB	teensy30_analog_output_meets_205_ac2	CI-enforced
AC#3	Pass-2 reconciliation logic	unit tests in library-select	CI-enforced
AC#4	stm32f103c8 auto-discovers SPI	stm32f103c8_blink_with_spi_auto_discovers_library_205_ac4	CI-enforced
AC#5	Warm cache hit <= 50 ms (bench-fastled-examples)	bench-fastled-examples Phase 7	CI-enforced
AC#6	Cold resolve on teensy41 <= 200 ms	teensy41_cold_library_selection_meets_205_ac6 (this PR)	CI-enforced after merge

Closes #205

Test plan

• bench-205.yml / acceptance-205.yml runs on this PR and the new teensy41 cold resolver matrix entry passes on Ubuntu CI
• Pre-existing acceptance gates (teensyLC, teensy30, stm32) remain green
• Clippy -D warnings clean on fbuild-build (verified locally)

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📒 Files selected for processing (2)

• .github/workflows/acceptance-205.yml
• crates/fbuild-build/tests/teensy41_acceptance.rs

✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch feat/205-ac6-teensy41-cold-resolver-gate

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