fix(android): adaptive tableau fan fraction fills portrait viewport

On a 360 dp portrait phone the card width is set by the 9-column
horizontal packing (360/9 = 40 dp); the fixed 0.25 fan fraction then
places the worst-case 13-card column in the top ~44 % of the screen,
leaving the bottom 56 % empty black.

`compute_layout` now solves for the fan fraction that exactly uses the
available vertical space below the tableau row:

    ideal = avail / (12 * card_height)

On height-limited (desktop) windows ideal ≈ 0.25 and the clamp to the
minimum keeps existing behaviour. On width-limited (portrait phone)
windows the fan expands — ≈ 0.84 at 360 × 800 dp — stretching the
tableau to fill the screen.

Both `tableau_fan_frac` and `tableau_facedown_fan_frac` (scaled
proportionally) are stored on the `Layout` struct. `card_plugin` and
`input_plugin` read from the struct so rendering and hit-testing stay
in sync at every viewport size.

Three new regression tests:
- portrait phone expands fan_frac beyond desktop minimum
- expanded fan fits inside phone viewport (no overflow)
- desktop fan_frac stays at minimum 0.25

Closes P1 "Portrait-first card spacing" in PLAYABILITY_TODO.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/android/PLAYABILITY_TODO.md

@@ -84,8 +84,17 @@ rewrites required.
   Material's guideline applies to all input modes. Cards, pile
   markers, modal close buttons not yet audited — track as P3 if
   they fall below threshold on hardware.
-- [ ] **Portrait-first card spacing.** Stretch tableau piles vertically
-  to fill height; reduce inter-pile gaps so 7 columns fit in 360 dp.
+- [x] **Portrait-first card spacing.** *Closed 2026-05-11.*
+  `compute_layout` now derives an adaptive `tableau_fan_frac` from the
+  available vertical space below the tableau row. On height-limited
+  (desktop) windows the formula returns ≈ 0.25 and the clamp keeps the
+  existing behaviour. On width-limited (portrait phone) windows — where
+  card size is constrained by the 9-column horizontal packing — the fan
+  fraction expands to fill the viewport (≈ 0.84 at 360 × 800 dp).
+  `tableau_facedown_fan_frac` scales proportionally. Both values live in
+  the `Layout` struct; `card_plugin::card_positions` and
+  `input_plugin::card_position` / `pile_drop_rect` read from the struct
+  so rendering and hit-testing stay in sync across viewport sizes.
 - [ ] **Double-tap auto-move visible feedback.** `handle_double_tap`
   exists since `395a322` — verify it triggers on hardware and add a
   brief source-card flash / highlight to confirm to the user.