fix(acceptance): #223 stm32 SPI + teensyLC LTO-symbol probes

[zackees]

Two pre-existing acceptance-205.yml gates failed for the same reason: under each orchestrator's Release profile (-flto -Os + --gc-sections) the test was probing for a symbol the linker had inlined and stripped. Both fixes broaden the probe to also accept an LTO-stable signal that proves the intended code actually linked.

stm32f103c8 SPI auto-discovery — closes #223

• crates/fbuild-build/tests/stm32_acceptance.rs:99 — accept either the mangled _ZN8SPIClass* symbol (non-LTO) or a PinMap_SPI_* global from Arduino_Core_STM32/libraries/SPI/src/utility/spi_com.c (LTO; address-taken const array survives --gc-sections).
• Orchestrator wiring is unchanged — already correct since #205 follow-up: wire library-selection cache into orchestrators (teensy, stm32) #214 (commit 86bb2d9).

teensyLC blink — acceptance-205.yml baseline failure since 2026-04-25

Same root-cause family: tiny setup() / loop() are visible in the LTO unit alongside Teensyduino's main.cpp and get inlined into main, with both unmangled and mangled symbols stripped. The original assertion (setup or _Z5setupv) is incompatible with the orchestrator's Release LTO settings.

• crates/fbuild-build/tests/teensylc_acceptance.rs:88 — accept any of three signals: unmangled setup / loop, mangled _Z5setupv / _Z4loopv, or the sketch's Serial.println("Teensy LC Test - LED Blink") literal present in the firmware bytes (strings in .rodata survive --gc-sections because their address is taken by the println call).
• crates/fbuild-build/tests/teensylc_acceptance.rs:114 — use result.compile_database_path from BuildResult instead of walking params.build_dir. The pipeline ignores params.build_dir and roots its cache at <project_dir>/.fbuild/build/<env>/<profile>/, so the previous walk-based lookup never found compile_commands.json. The orchestrator already reports the effective path — trust it.

Test plan

• uv run soldr cargo test -p fbuild-build --release --test stm32_acceptance -- --ignored stm32f103c8_blink_with_spi_auto_discovers_library_205_ac4 passes locally
• uv run soldr cargo test -p fbuild-build --release --test teensylc_acceptance -- --ignored teensylc_blink_meets_205_acceptance_criteria passes locally
• uv run soldr cargo clippy --workspace --all-targets -- -D warnings clean
• uv run soldr cargo fmt --all clean
• CI: acceptance-205.yml STM32 gate goes green
• CI: acceptance-205.yml teensyLC gate goes green (first time green since 2026-04-25)

Closes #223

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• crates/fbuild-build/tests/stm32_acceptance.rs
• crates/fbuild-build/tests/teensylc_acceptance.rs

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• Commit unit tests in branch fix/223-stm32-spi-auto-discovery

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