feat: on-device history + MINMAX/DAYCOMP display modes

[varffs]

Adds on-device history to the polytunnel monitor and two new 16x2 screens driven by it. The device can now answer "how cold did it get last night, and was that worse than the night before?" without reaching for a phone.

What's here

• MINMAX — rolling 24 h min/max, internal and external temperature
• DAYCOMP — today's internal min/max above yesterday's
• Button cycle is now DEFAULT → MINMAX → DAYCOMP → DIAG

Backed by an in-memory ring of sensor snapshots pruned to 49 h, with all display values derived by pure selectors at render time. One history/record action is dispatched per poll cycle after both sensors settle; the reducer composes the sample from state.data and the payload carries only the timestamp, so tests drive time without mocking the clock.

Deliberate non-goals

Settled during design, recorded so they don't get re-litigated:

• No persistence of any kind. No database, no JSON snapshot, no append log. 288 samples/day doesn't justify SQLite, and a native module would add an armv6 build step to every deploy. History resets on restart.
• No dashboard, no query API, no iotplotter backfill.
• The ring lives in the app, not in piteknix — promotion waits for a real second consumer (the bathroom device).

Accepted consequences: DAYCOMP shows yst L-- H-- until the device crosses a midnight, and for the first hour after a restart the "24 h" window holds only samples since boot.

Two things worth a reviewer's attention

Whole degrees, not one decimal. The original spec drew i24 L-3.2 H31.4 and reasoned about a worst case of "negative min, positive max". That's wrong: the real worst case is an all-day freeze where both values are two-digit negative, and tdy L-12.4 H-10.1 is 17 characters on a 16-column display. Whole degrees fit with margin and keep the L/H labels. DEFAULT still shows the live reading to one decimal.

A pre-existing sensor bug this feature would have amplified. pollInternal caught a failed read and dispatched nothing, so state kept the last good value. That was a momentary wrong reading on one screen; with a history ring it becomes 288 fabricated samples a day that keep answering MINMAX/DAYCOMP for 24–48 h after the sensor recovers — silently flattening an outage into a plausible-looking night. Fixed here: the catch now nulls both fields, and formatDataLines guards line 0 so a null internal renders int: -- -- rather than int: 0c 0%.

Also fixed: the retention window is 49 h rather than 48 h, because on the evening a DST fall-back lengthens a local day, DAYCOMP's "yesterday" starts up to 48 h 59 m before now.

Verification

56/56 tests passing. Everything added is pure — no hardware, no filesystem, no clock mocking.

Full mode cycle exercised in virtual mode end to end:

DEFAULT   |int: 23.0c 66.8%|
MINMAX    |i24 L23 H23     |
DAYCOMP   |tdy L23 H23     |   yst L-- H-- (no midnight crossed)
DIAG      |ext: ok         |

Not verified: the dark→wake button transition (boot leaves the display lit, so the driven run never started dark) — covered by unit tests rather than by that run. Nothing has run on the real Pi yet.

Design: docs/superpowers/specs/2026-07-26-history-display-modes-design.md
Plan: docs/superpowers/plans/2026-07-26-history-display-modes.md

Follow-ups, not in this PR

• Stamp v2.0.0 on release (package.json is still 0.1.0)
• Deploy to the Pi — remember git checkout -- package-lock.json before pulling on-device
• formatDataLines still emits ext: -- (unknown) at 17 chars — pre-existing, untouched

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds an in-memory, on-device history ring to the polytunnel monitor and introduces two new LCD display modes (MINMAX, DAYCOMP) driven by pure selectors/formatters, plus fixes an internal-sensor failure case to avoid silently reusing stale readings.

Changes:

• Add history.samples ring buffer recorded once per poll cycle, pruned to a 49h window and hard-capped for clock anomalies.
• Introduce pure selectors (src/history.js) and new render modes/formatters (MINMAX, DAYCOMP) with whole-degree summaries.
• Change sensor failure behavior so a failed internal read explicitly dispatches nulls (and update tests/docs accordingly).

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/store.js	Adds history slice, ring retention/cap constants, record action, and expands display mode cycle to 4 modes.
src/store.test.js	Updates mode-cycle test and adds reducer-level history recording/pruning/capping tests plus a MINMAX integration-style assertion.
src/sensors.js	On internal sensor read failure, dispatches null temperature + humidity instead of dispatching nothing.
src/sensors.test.js	Updates internal-sensor fault test to assert null dispatches.
src/history.js	Adds pure time-window, min/max, and local-day bucketing selectors.
src/history.test.js	Adds unit tests for selector windowing, null-skipping, and local-midnight day separation.
src/render.js	Adds MINMAX/DAYCOMP formatters and threads an injectable now through renderDisplay; adjusts DEFAULT formatting for null internal temp.
src/render.test.js	Adds tests for new formatters/modes and updates DEFAULT placeholder expectation; verifies 16-char max for new lines.
src/backlight.test.js	Updates expectations to match the new mode order after button presses while lit.
app.js	Records one history sample per poll cycle and triggers re-render on history/* actions.
CHANGELOG.md	Documents new history display modes and internal sensor failure behavior fix.
docs/superpowers/specs/2026-07-26-history-display-modes-design.md	Adds design doc describing data model, retention rationale, selectors, and display behavior.
docs/superpowers/plans/2026-07-26-history-display-modes.md	Adds implementation plan for the history ring and new display modes.

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[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated 1 comment.