fix(ldf): avoid self-selecting Teensy framework libraries and their global state

[zackees]

Regression

A Teensy 4.1 Blink sketch that only includes FastLED still has framework Audio, SD, SdFat, and SerialFlash selected by fbuild lib-select, even after FastLED removes its Audio.h LDF hint and all source includes of Audio.h / SD.h.

fbuild lib-select .build/pio/teensy41 -e teensy41 --explain reports:

• Audio triggered by the framework's own libraries/Audio/AudioControl.h
• SD triggered by the framework's own libraries/SD/src/SD.cpp
• SdFat triggered by the framework's own libraries/SdFat/src/DigitalIO/DigitalPin.h
• SerialFlash selected transitively

This is a self-discovery cycle: once the resolver reaches library sources/headers, headers in that same library appear to be treated as new roots. The result is all of the library sources being compiled. SD.cpp contains the global SDClass SD, which survives LTO and contributes 1,236 B .bss to Blink.

Reproduction

1. Check out FastLED branch implementing #3696 (no <Audio.h> in src/platforms/arm/teensy/ldf_headers.h).
2. bash compile teensy41 --examples Blink
3. fbuild lib-select .build/pio/teensy41 -e teensy41 --explain
4. fbuild symbols .build/pio/teensy41/.fbuild/build/release/firmware.elf --top 100 --no-graph

Expected: only libraries reachable from the sketch/FastLED roots are selected; no Audio/SD/SdFat/SerialFlash for Blink.

Actual: the resolver selects 11 libraries including Audio (91 sources), SD, SdFat (46 sources), and SerialFlash. The linked ELF retains SD (1,236 B) and DMAChannel::begin despite the sketch not using any of them.

Acceptance

• Do not use a library's own transitive headers/sources as a root that selects that same library.
• A fresh resolver run for FastLED Blink on teensy41 does not select Audio, SD, SdFat, or SerialFlash.
• The resulting Blink ELF contains none of their symbols/global state.
• Preserve selection when the sketch or FastLED actually includes a feature-specific header.

Related: FastLED/FastLED#3696; historical fbuild #204 and #1088.

[zackees]

New selector evidence from FastLED/FastLED#3696:\n\nbuild lib-select .build/pio/teensy41 -e teensy41 --explain on the Blink sketch reports 2,254 source files scanned / 2,694 headers reached and self-selects framework extras:\n\n- Audio — triggered by that candidate library's libraries/Audio/AudioControl.h\n- SD — triggered by libraries/SD/src/SD.cpp\n- SdFat — triggered by libraries/SdFat/src/DigitalIO/DigitalPin.h\n- SerialFlash — triggered by libraries/SerialFlash/SerialFlash.h\n\nBlink does not include those headers. The required resolver model is sketch-rooted transitive discovery: begin at the sketch and its reachable FastLED includes; inspect a candidate library only after a reachable include resolves to it. A candidate's own headers/sources must never nominate itself.\n\nFastLED moved its generic Arduino SD implementation out of the public header graph as hygiene, but the selection remains because this is framework-library self-selection. build symbols still reports external SD/SdFat symbols in Blink, so fixing this resolver behavior is required to remove the residual bloat.

[zackees]

Implemented in #1097. Local live fbuild lib-select on FastLED Teensy Blink now starts from two translation units rather than recursively seeding lib/FastLED; Audio, SD, SdFat, and SerialFlash are absent from the selected libraries. The remaining optional DmxSimple/OctoWS2811/SoftwareSerial selections come from intentionally preprocessor-insensitive conditional scanning and are tracked separately from this self-selection fix.