build: declare ESP toolchain in mise.toml; drop ~/export-esp.sh dependence

mise.toml now owns the ESP cross-toolchain end to end:

- [env] pins PATH (xtensa-esp-elf-gcc bin) and LIBCLANG_PATH so any
  `mise exec` (which the justfile already uses for `set shell`)
  resolves the linker without sourcing espup's stray ~/export-esp.sh.
- [tasks.esp-install] installs the matching crosstool version via
  espup (espup hardcodes ~/.rustup/toolchains/esp/ -- there is no
  --prefix flag -- so the binaries necessarily live there, but the
  project now declaratively owns the version + path knowledge).
- cargo:espup pinned to 0.17.1 to match the [env] paths.

Triggered by a real failure: ~/export-esp.sh was moved to Trash and
the ESP build broke with `linker xtensa-esp32s3-elf-gcc not found`.
The previous setup silently no-op'd `[ -f $HOME/export-esp.sh ] && .
$HOME/export-esp.sh` in justfile, leaving the build with no PATH for
the cross-toolchain. Encoding the paths in mise.toml removes the
stray-file dependency.

justfile changes:
- _cargo: drop the `[ -f $HOME/export-esp.sh ] && . ...` source.
- setup / _require_esp_toolchain: install via `mise run esp-install`
  instead of bare `espup install`, so the version pin and the [env]
  paths stay in sync from a single source.

Bumping ESP versions remains a two-touch change: edit the version
dirs in BOTH [tasks.esp-install] and [env], then re-run the task.

justfile

@@ -19,10 +19,11 @@ boards := "heltec_v3 heltec_v3_uart heltec_v4 elecrow_thinknode_m2 lilygo_tbeam
 setup:
     mise trust --yes
     mise install
-    @# ESP Xtensa toolchain (not a rustup component — installed out-of-band by espup)
+    @# ESP Xtensa toolchain — installed via the `esp-install` mise task
+    @# (which pins versions to match mise.toml's [env] paths).
     @if ! rustup toolchain list | grep -q "^esp"; then \
-        echo "ESP toolchain not found, installing via espup (this may take a while)..."; \
-        espup install || { echo "error: espup install failed" >&2; exit 1; }; \
+        echo "ESP toolchain not found, installing via mise task (this may take a while)..."; \
+        mise run esp-install || { echo "error: esp-install task failed" >&2; exit 1; }; \
     fi
 
 # Build release firmware for all boards, installing toolchains as needed
@@ -154,7 +155,6 @@ _cargo board cmd extra_args="" extra_features="":
     extra=""; buildstd=""; \
     case "$target" in xtensa-*) \
         just _require_esp_toolchain; \
-        [ -f "$HOME/export-esp.sh" ] && . "$HOME/export-esp.sh"; \
         extra="+esp"; buildstd="-Zbuild-std=core,alloc";; \
     esac; \
     cargo $extra {{cmd}} --target "$target" --features "$feat" $buildstd {{extra_args}}
@@ -168,15 +168,17 @@ _can_build board:
         *) rustup target list --installed | grep -q "^$target$" || rustup target add "$target" ;; \
     esac
 
-# Ensure the ESP Xtensa toolchain is installed (espup installed via mise).
-# The esp toolchain ships with its own cargo, rustc, and rust-src — no extra
+# Ensure the ESP Xtensa toolchain is installed. The actual install is
+# delegated to the `esp-install` mise task so the version pin lives in
+# mise.toml alongside the matching [env] PATH/LIBCLANG_PATH entries.
+# The esp toolchain ships its own cargo, rustc, and rust-src — no extra
 # components are needed on top.
 [private]
 _require_esp_toolchain:
     @just _ensure_tools
     @if ! rustup toolchain list | grep -q "^esp"; then \
-        echo "ESP toolchain not found, installing via espup (this may take a while)..." >&2; \
-        espup install || { echo "error: espup install failed" >&2; exit 1; }; \
+        echo "ESP toolchain not found, installing via mise task (this may take a while)..." >&2; \
+        mise run esp-install || { echo "error: esp-install task failed" >&2; exit 1; }; \
     fi
 
 # Find a serial port matching any of the given VID:PID pairs (checked in order)

mise.toml

@@ -4,7 +4,7 @@ just = "latest"
 # Cargo-installed tooling via mise's `cargo:` backend.
 "cargo:cargo-binutils" = "latest"   # rust-objcopy, rust-size
 "cargo:cargo-hex-to-uf2" = "latest" # ELF → UF2 for nRF52 / RP2040 DFU drives
-"cargo:espup" = "latest"            # bootstraps the ESP Xtensa toolchain
+"cargo:espup" = "0.17.1"            # ESP Xtensa toolchain installer (pinned: [env] paths below match this version's defaults)
 "cargo:espflash" = "3"              # flash + save-image for ESP targets (4.x requires esp_app_desc!() in app — revisit alongside esp-hal 1.1+ / drop-the-patches cleanup)
 "cargo:probe-rs-tools" = "latest"   # debug-probe flash/run for ARM targets
 
@@ -13,7 +13,7 @@ just = "latest"
 # extra `rustup target add` step at setup time.
 #
 # Xtensa ESP targets are NOT listed here — they use the self-contained `esp`
-# toolchain installed out-of-band by `espup` (see `just setup`). That
+# toolchain populated by `mise run esp-install` (see [tasks] below). That
 # toolchain ships its own rust-src for `-Zbuild-std`.
 #
 # profile = "minimal" (just rustc, rust-std, cargo), "default" (adds rust-docs, rustfmt, clippy)
@@ -22,3 +22,25 @@ just = "latest"
 version = "1.94"
 profile = "default"
 targets = "thumbv6m-none-eabi,thumbv7em-none-eabihf"
+
+# ESP Xtensa toolchain. `cargo:espup` above only installs the *installer*;
+# the actual rustc-esp + xtensa-esp-elf-gcc + esp-clang toolchains are
+# populated by `mise run esp-install` (or `just setup`).
+#
+# espup hardcodes its install root to ~/.rustup/toolchains/esp/ — there's
+# no --prefix or --install-dir flag — so the binaries necessarily live
+# there. The [env] block below pins the exact PATH and LIBCLANG_PATH so
+# every `mise exec` (which the justfile uses for `set shell`) resolves
+# the cross-toolchain without depending on espup's stray ~/export-esp.sh.
+#
+# Bumping versions: edit the version dirs in BOTH [tasks.esp-install] (so
+# espup installs the matching crosstool) AND [env] (so the path resolves).
+[env]
+_.path = [
+  "{{env.HOME}}/.rustup/toolchains/esp/xtensa-esp-elf/esp-15.2.0_20250920/xtensa-esp-elf/bin",
+]
+LIBCLANG_PATH = "{{env.HOME}}/.rustup/toolchains/esp/xtensa-esp32-elf-clang/esp-20.1.1_20250829/esp-clang/lib"
+
+[tasks.esp-install]
+description = "Install the ESP Xtensa toolchain (rustc-esp + GCC + clang) at versions matching [env]"
+run = "espup install --crosstool-toolchain-version esp-15.2.0_20250920"