Phase 0b-i (#491): emit firmware.map next to firmware.elf for ESP32 link #508

Part of #491 (firmware.map auto-emit), narrowed to ESP32 because that's the target for Phase 4 of #493. Other platforms missing the flag (Apollo3, NXP LPC, RP2040) can be addressed in follow-up sub-issues.

Background

crates/fbuild-build/src/esp32/esp32_linker.rs builds the ESP32 link command but does not pass -Wl,-Map=…/firmware.map. Every other platform that ships through fbuild's own linker layer (avr_linker.rs, teensy_linker.rs, generic_arm/arm_linker.rs covering STM32/NRF52, esp8266_linker.rs, ch32v_linker.rs, renesas_linker.rs, sam_linker.rs, silabs_linker.rs, nrf52_linker.rs) already does. ESP32 is the outlier.

This blocks fbuild bloat / fbuild symbols from filling the ARCHIVE / OBJECT / SECTION columns on ESP32 firmware (and therefore the "flash bytes by archive" rollup), which is a hard prerequisite for Phase 4 of #493 (the shrink integration that needs archive attribution to report "libc.a/lib_a-vfprintf.o" deltas).

Scope

• crates/fbuild-build/src/esp32/esp32_linker.rs: after the existing -o firmware.elf argv push, also append -Wl,-Map=<output_dir>/firmware.map. Match the pattern used in teensy_linker.rs:94, avr_linker.rs:112, etc.
• Regression test analogous to the existing teensy/AVR tests:
  • Build the link argv via the existing test fixture.
  • Assert the argv contains a -Wl,-Map= token that resolves to firmware.map in the same directory as firmware.elf.

Out of scope

• Apollo3, NXP LPC, RP2040 — separate sub-issues.
• build_info.json recording the map path — already covered by fbuild symbols: ingest toolchain paths from build_info.json + extend schema with nm/c++filt/readelf/objdump #428 (existing infrastructure).
• Auto-emit of build_info.json itself — Phase 4 of fbuild build: --shrink[=MODE] (default auto) for per-platform flash reduction via spec-file printf swap #493 wires this end-to-end.

Pass

• cargo build --workspace --all-targets clean.
• cargo clippy --workspace --all-targets -- -D warnings exits 0.
• New unit test passes.
• After a clean fbuild build tests/platform/esp32s3 -e esp32s3, firmware.map sits next to firmware.elf (smoke-verifiable manually; CI doesn't run full ESP32 builds).