feat(layout): adaptive layout & font scaling system for plugins

Add src/adaptive_layout.py — opt-in core helpers so plugins render
legibly on any panel size without hand-tuned per-display layouts:

- Region: integer rect algebra (bands/columns/weighted splits/centering)
  that partitions space so text bands can't overlap by construction
- Font ladders: ordered (family, size) steps known to render crisply
  (LADDER_GRID: X11 BDFs at native sizes; LADDER_ARCADE: PressStart2P at
  8px multiples) — fitting walks the ladder instead of scaling pixel
  fonts fractionally
- LayoutContext: breakpoint tiers, geometry scale vs. a declared design
  size, and cached fit_text/fit_lines/font_for_rows queries

Generalizes the three patterns proven in the field: f1-scoreboard's
scale factor, masters-tournament's tiers, baseball-scoreboard's font
fallback ladder.

Wiring: BasePlugin gains a lazy .layout property and draw_fit();
FontManager gains get_native_bdf_size() and a cache_generation counter;
manifest schema gains display.design_size and requires.display_size
max_width/max_height; 96x48 joins DEFAULT_TEST_SIZES; the bounds-check
harness records negative-coordinate draws; TextHelper's broken
measurement helpers are fixed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/ADAPTIVE_LAYOUT.md

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+# Adaptive Layout & Font Scaling
+
+`src/adaptive_layout.py` lets a plugin render legibly on **any** panel size
+(64x32, 128x32, 96x48, 128x64, 256x64, ...) without hand-tuned per-display
+layouts. It is **opt-in**: nothing changes for plugins that don't use it.
+
+It generalizes three patterns proven in the plugin ecosystem:
+
+| Pattern | Origin | Core API |
+|---|---|---|
+| Geometry scale factor vs. a design size | f1-scoreboard | `ctx.px(base)` / `ctx.scale` |
+| Breakpoint tiers | masters-tournament | `ctx.tier` / `ctx.by_tier({...})` |
+| "Largest crisp font that fits" ladder | baseball-scoreboard | `ctx.fit_text(...)` and friends |
+
+## Quick start
+
+Every `BasePlugin` has a lazy `self.layout` (a `LayoutContext` for the
+current logical display size, rebuilt automatically if the size changes)
+and a one-liner `self.draw_fit(...)`:
+
+```python
+def display(self, force_clear=False):
+    from src.adaptive_layout import LADDER_ARCADE
+
+    b = self.layout.bounds.inset(1)          # Region(0,0,W,H) minus 1px margin
+    rows = b.split_v(3, 1, 1, gap=1)          # 3/5 for time, 1/5 each for the rest
+
+    self.draw_fit(self.time_str, rows[0], ladder=LADDER_ARCADE)
+    self.draw_fit(self.weekday, rows[1])      # default LADDER_GRID
+    self.draw_fit(self.date_str, rows[2])
+    self.display_manager.update_display()
+```
+
+On 128x64 the time renders at press_start 24px; on 64x32 it steps down to
+8px. The rows partition the height, so bands can never overlap — no more
+`y = height - 7` magic numbers.
+
+## Region — rect algebra
+
+`Region(x, y, w, h)` is a frozen dataclass. All carving clamps to
+non-negative dimensions, so degenerate panels behave.
+
+- Carving: `inset(dx, dy)`, `top_band(h)`, `bottom_band(h)`,
+  `middle(top_h, bottom_h)`, `left_col(w)`, `right_col(w)`,
+  `split_h(*weights, gap=0)`, `split_v(*weights, gap=0)`
+- Placement: `align_xy(w, h, align, valign)`, `center_xy(w, h)`,
+  `contains(w, h)`, `.center`, `.right`, `.bottom`
+
+Scoreboard-style layout:
+
+```python
+b = self.layout.bounds
+status = b.top_band(self.layout.px(7))
+detail = b.bottom_band(self.layout.px(7))
+score_area = b.middle(status.h, detail.h)
+away_slot, home_slot = b.left_col(b.h), b.right_col(b.h)
+```
+
+## Font ladders — discrete, never fractional
+
+Pixel fonts (BDF, PressStart2P) only look right at native/integer sizes, so
+fonts are never scaled continuously. A `FontLadder` is an ordered tuple of
+`FontStep(family, size_px)` rungs, largest first; fitting walks down until
+the measured text fits.
+
+- `LADDER_GRID` (default): X11 BDFs at native sizes — 10x20 → 9x18 → 9x15 →
+  8x13 → 7x13 → 6x13 → 6x12 → 6x10 → 6x9 → 5x8 → 5x7 → 4x6 → tom-thumb.
+  Body text, labels, multi-row content.
+- `LADDER_ARCADE`: PressStart2P at 32/24/16/8 (integer multiples of its 8px
+  grid). Headline text: clocks, scores.
+
+Custom ladders are just tuples — e.g. to add your plugin's registered font
+on top: `(FontStep("myplugin::digits", 16),) + LADDER_GRID`.
+
+## LayoutContext
+
+Built per (width, height); exposes facts and fit queries:
+
+- `bounds`, `width`, `height`, `aspect`
+- `tier` by height (`xs`≤16, `sm`≤32, `md`≤48, `lg`≤64, `xl`) and
+  `width_tier` (`narrow`≤64, `normal`≤128, `wide`≤256, `ultrawide`)
+- `is_wide_short` — aspect ≥ 2.5 and height ≤ 32 (the classic 128x32 shape)
+- `scale` — `min(w/design_w, h/design_h)` vs. your manifest's
+  `display.design_size` (default 128x32). **Geometry only** — gaps, icon
+  and logo sizes via `px(base, minimum, maximum)`; fonts use ladders.
+- `by_tier({"sm": 10, "lg": 18})` — value for the nearest defined tier
+  at-or-below the panel's tier.
+- `fit_text(text, box, ladder, ellipsis=True)` → `FitResult` — largest rung
+  that fits; ellipsizes as a last resort. Cached per (text, box, ladder).
+- `fit_lines(lines, box, ladder, spacing)` — every line fits the width and
+  the stack fits the height (measures the actual strings).
+- `font_for_rows(rows, box_h, ladder)` — largest rung whose line height
+  fits `rows` rows.
+
+`FitResult` carries the ready-to-use `font` (drops straight into
+`display_manager.draw_text(font=...)`), the possibly-ellipsized `text`,
+ink `width`/`height`, `baseline`, `y_offset`, `line_height`, and `fits`.
+
+## Manifest declaration
+
+Declare the size your layout was authored against so `ctx.scale` means
+something:
+
+```json
+"display": { "design_size": { "width": 128, "height": 32 } }
+```
+
+Also available under `requires.display_size`: `min_width`, `min_height`,
+`max_width`, `max_height`.
+
+## Performance notes (Pi)
+
+Fit queries are cached, so cost is O(unique strings). For per-second text
+(clocks, live scores), fit on a **shape placeholder** and reuse the font:
+
+```python
+fit = self.layout.fit_text("00:00", box, ladder=LADDER_ARCADE)  # cached once
+self.display_manager.draw_text(current_time, font=fit.font, ...)
+```
+
+## Testing across sizes
+
+The harness already renders every plugin at a spread of sizes (now
+including 96x48):
+
+```bash
+python scripts/check_plugin.py <plugin-dir> --sizes 64x32,128x32,96x48,128x64,256x64
+python scripts/render_plugin.py <plugin-dir> --width 96 --height 48
+```
+
+`BoundsCheckingDisplayManager` flags right/bottom overflow and now records
+mediated draw calls with negative coordinates in
+`negative_coordinate_calls` (raw-PIL draws remain uncovered).
+
+Reference migration: the **text-display** plugin's `font_mode: "auto"`.