Integrated scale support for Bengle

[ChampionDesigns]

Filed inside the fork so the diff shows only this PR's own changes.
It will be re-filed against decentespresso/de1app once its base has merged there.

merges. 160 insertions, 9 files.

Summary

A Bengle has a scale in the machine, with no BLE peripheral and no handle. Its weight and
gravimetric flow arrive on 0xA013. Every scale check in the app is really a check for a
paired BLE address, so on a Bengle the weight readouts return empty, the stop-at-weight
slider is hidden, and the timer falls back to volumetric text while a working scale streams
the whole time.

The important detail

_apply_shotvalue feeds Weight into ::device::scale::process_weight_update, the same
entry point all fourteen BLE scale drivers use. That pipeline owns the weight history, the
filtered weight and flow estimates, the scale_stop_at_half_shot two-cup scaling,
tare-completion detection, the watchdog, the app-side stop-at-weight check and drink-weight
recording.

Setting ::de1(scale_weight) directly instead — which an earlier draft did — writes the
pipeline's outputs and skips all of it.

GFlow, the firmware's own gravimetric flow, is not used for scale_weight_rate. The
pipeline derives flow from the weight history as it does for every other scale. GFlow is
kept on ::de1(integrated_scale_flow) for the comparison chart; switching to the firmware
estimate wants a bench comparison first.

Changes

device_scale.tcl — is_connected returns true when the v2 protocol is active, which
is the honest answer: the scale is present and reporting, and there is no handle because
there is no peripheral. And ::device::scale::tare selects on scale_type, which a Bengle
does not have, so the Bengle case is handled before the switch and writes ScaleTare
(0x0080388C). That matters more than a tare button: tare is called automatically before
every espresso and hot-water pour when a cup is on the platform, from the HotWater state
handler, and from any profile step whose message contains "tare".

de1_comms.tcl — set_bengle_scale_tare, and set_end_of_shot_weight, which writes
the per-profile target to EndOfShotWeight (0x00803864, grams ×100, clamped 0–1000000)
on every frame upload. Writing 0 clears one left on disk by a previous profile.

gui.tcl, utils.tcl, shot.tcl, vars.tcl — three chart vectors recording the
integrated series beside the existing ones, and the weight readouts and pour-timer text so
the volumetric fallback applies only when there is no scale at all.

skins/default/de1_skin_settings.tcl — the stop-at-weight block is shown for a Bengle
as well as for a paired BLE scale.

Visualizer payload

On v1 nothing changes. On v2 the integrated series fill by_weight and weight, and a
paired BLE scale is emitted as by_weight_external / weight_external rather than dropped.
by_weight_raw is emitted on both, as today.

One value is not verified

::device::scale::sensor_lag selects on scale_type, so a Bengle falls to the 0.38 s
default meant for a BLE scale. An integrated scale reporting on the machine's own sample
stream should be lower. Left at the default rather than guessed. TODO (Ben): measure it.

Checked against decaid

decentespresso/decaid implements the same feature and agrees on the wire: the
EndOfShotWeight register, scale and clamp are identical, and ScaleTare takes the same
value at the same address. It reaches the result differently — it exposes the integrated
scale as a real Scale device, so the pipeline runs with no special case. This PR keeps the
app's existing shape and joins the pipeline at process_weight_update.

Impact on a DE1

is_connected falls through to the handle test. The bridge never runs. The tare branch is
not taken. The three vectors stay empty. set_end_of_shot_weight returns early. The
Visualizer key set is identical to main's.

Test plan

• The v1 Visualizer payload key set is unchanged against main.
• Pull a shot on a DE1 with a BLE scale paired. Confirm weight, flow-by-weight and
  stop-at-weight behave as before.
• TODO (Ben): pull a shot on a Bengle with no BLE scale. Confirm the weight readout is
  live, the slider is shown, and the shot stops on target.
• TODO (Ben): pull a shot on a Bengle with a BLE scale also paired.
• TODO (Ben): put a cup on during warm-up. Confirm the automatic tare fires.
• TODO (Ben): with scale_stop_at_half_shot on, confirm the reading doubles.
• TODO (Ben): measure the integrated scale's sensor lag.

Testing

Manual verification only. There is no automated test coverage for these Tcl paths in the
repo.