feat(deploy): preserve app data on --native when signing key matches

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Problem

mix mob.deploy --native wiped on-device app data — MOB_DATA_DIR (the app's
identity, built-screen stores, anything in files/) — on every native
deploy. adb_install_all/3 ran an unconditional adb uninstall <pkg> before
adb install <apk>, so even an in-place update of an APK signed with the same
key (e.g. a committed debug keystore pinned precisely to avoid this) lost its data.

This makes a stable on-device identity impossible across --native redeploys: a
committed keystore stabilizes beam-only mix mob.deploy, but the native path
threw the data away regardless.

Root cause (verified empirically)

On one device, same keystore-signed APK:

Operation	App data
mix mob.deploy --native	wiped (firstInstallTime reset)
manual adb install -r <apk>	preserved (firstInstallTime unchanged)

So install -r works fine — the explicit uninstall was the cause, not any
signature mismatch.

Fix

adb_install_all/3 now attempts adb install -r first (an in-place update that
preserves data when the signing key matches) and only falls back to
uninstall + install when the in-place update is genuinely rejected — a
non-zero exit or an INSTALL_FAILED_* line (INSTALL_FAILED_UPDATE_INCOMPATIBLE
for a real signature change, INSTALL_FAILED_VERSION_DOWNGRADE, etc.). When it
does fall back, it logs that app data will be cleared.

This strictly improves on the old behavior (which always uninstalled): the
fallback path is identical, it's just no longer the default.

The decision is a pure helper, NativeBuild.needs_clean_reinstall?/2, unit-tested
for the success / signature-mismatch / downgrade / non-zero-exit cases.

Tests

test/mob_dev/native_build_test.exs — 4 new cases; full file 156 passed.
Pre-push (format + credo + suite) green.

Docs

CHANGELOG.md [Unreleased] → Changed, so users can track the behavior change.

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