B4: src/ include/ lib/ layout - behaviour-neutral #186

_{Recreated from Strycher/Field_Compass-archive#186 — originally opened by @Strycher on 2026-08-09. Credentials redacted where present.}

Epic: #154

Adopt PlatformIO's layout. No logic change - the .ino still compiles as-is. Splitting it into translation units is explicitly deferred (see the plan's Deferred table).

Scope

• src/ for the sketch, include/ for headers (already holds lv_conf.h), lib/ for vendored libraries
• Field_Compass/lv_psram_alloc.c (build: main no longer links — DRAM overflowed by 4296 bytes #164) must move with it and still compile. Verify the LVGL CUSTOM allocator actually links - a missing core function fails at link, which is the loud failure we want
• Keep the arduino-cli path working in parallel if practical; if not, say so explicitly, because that fallback matters until B7: Epic B integration test — PlatformIO build on hardware #161 signs off

Acceptance criteria

• pio run produces a binary
• Flash and RAM figures unchanged from B3 - this task moves files, it does not change code