perf(deploy): make espflash a build-time-optional cargo feature

[zackees]

Summary

Touches issue #66 but not in the originally-requested direction. The espflash-based verify+write migration already landed in PRs #74 / #106 — the remaining observable cost is that every fbuild build pulls espflash (and its ~30 transitive deps: strum, deku, miette, esp-idf-part, ...) whether or not the operator ever sets FBUILD_USE_ESPFLASH_*.

This PR walks that back: espflash + md-5 become optional in fbuild-deploy, and a new espflash-native cargo feature on fbuild-deploy + fbuild-daemon gates them in.

Change

Default build (new behaviour):

• espflash, md-5, and their transitive deps are not compiled.
• esp32_native.rs is not compiled.
• FBUILD_USE_ESPFLASH_VERIFY / FBUILD_USE_ESPFLASH_WRITE are ignored; the daemon always takes the esptool-subprocess path.
• Sole trade-off: operators who relied on those env vars on a stock build need to rebuild with --features fbuild-daemon/espflash-native.

--features espflash-native build:

• Identical runtime behaviour to today's main. Env vars still pick native-vs-esptool.

Feature gates (what is cfg-gated)

• fbuild-deploy: pub mod esp32_native; Esp32Deployer::{use_native_verify, use_native_write} struct fields; Esp32Deployer::{with_native_verify, with_native_write} builders; try_verify_deployment_native, try_deploy_native helper fns. Native-path dispatch arms inside the public methods are also #[cfg(feature = "espflash-native")].
• fbuild-daemon: native_verify_enabled / native_write_enabled helper fns, plus the sole call site that wires with_native_{verify,write}.

Quality gates (verified locally on Windows 10)

• uv run cargo clippy --workspace --all-targets -- -D warnings — clean (default, no feature).
• uv run cargo clippy --workspace --all-targets --features fbuild-daemon/espflash-native -- -D warnings — clean.
• uv run cargo test -p fbuild-deploy — 36 pass, 0 fail, 8 ignored.
• uv run cargo fmt --all --check — clean.

Scope / Non-goals

• Does not remove any existing code paths.
• Does not drop esptool as a dep.
• Does not change the env-var contract (besides "only honoured when the feature is on").
• Does not attempt to measure the binary-size / build-time delta — that's for a follow-up once CI picks it up.

Test plan

• CI green across Linux / macOS / Windows.
• Build-time delta on the default profile is measurable vs current main (expect several seconds faster cold build, ~30 fewer crates downloaded).

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 4f4f640c-08fb-4344-8da7-66264ed7d0e0

📥 Commits

Reviewing files that changed from the base of the PR and between 6e03e9b and 32e11ab.

📒 Files selected for processing (5)

• crates/fbuild-daemon/Cargo.toml
• crates/fbuild-daemon/src/handlers/operations.rs
• crates/fbuild-deploy/Cargo.toml
• crates/fbuild-deploy/src/esp32.rs
• crates/fbuild-deploy/src/lib.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch spike/espflash-native-verify

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