feat: P.0.lib.input action-mapping + direction-vector helper

README.md

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+# lib-core.input
+
+P.0 action-mapping lib. Decouples physical keys from logical actions.
+Polling-based (`is_action_down`) + edge-based (`was_action_pressed`).
+Direction-vector helper composes 4 actions into `{x, y}` ∈ {-1, 0, +1}².
+
+- Lib-ID: `lib-core.input`
+- Version: `0.1.0`
+- Spec: `meta/docs/superpowers/specs/2026-05-09-p0-lib-input-design.md`
+
+Forward-compat stubs (DEPRECATED-MVP) for mouse-button actions, gamepad
+bindings, action-context-stack, key-rebinding-config, modifier-combos,
+analog-axis, was_action_released edge.
+
+## API
+- `input.bind(action_name, {key1, key2, ...})` — bind multi-key array
+- `input.unbind(action_name)` — remove binding
+- `input.is_action_down(action) → bool` — any bound key currently down
+- `input.was_action_pressed(action) → bool` — any bound key just-pressed
+- `input.direction(left, right, up, down) → {x, y}` — vec in {-1,0,+1}², Y-down-positive
+- `input.action_count() → number` — bound-actions count
+
+## Conventions
+- Y-down-positive per Sporel pixel-convention (ADR-0031).
+- Multi-key per action (e.g., `{KEY_A, KEY_LEFT}` for both A and Left-Arrow).
+- Silent-false for undefined actions (debug-friendly; lookup-misuse won't
+  drown the game-loop in errors).
+- Lua `error(...)` for `bind()` misuse (caller-bug fast-fail).
+
+## Consumer pattern
+```lua
+local input = require("lib-core.input")
+input.bind("quit",      { engine.input.KEY_ESCAPE })
+input.bind("pan_left",  { engine.input.KEY_A, engine.input.KEY_LEFT })
+input.bind("pan_right", { engine.input.KEY_D, engine.input.KEY_RIGHT })
+input.bind("pan_up",    { engine.input.KEY_W, engine.input.KEY_UP })
+input.bind("pan_down",  { engine.input.KEY_S, engine.input.KEY_DOWN })
+
+function update(ctx, dt)
+    if input.was_action_pressed("quit") then engine.exit(0) end
+    local d = input.direction("pan_left", "pan_right", "pan_up", "pan_down")
+    -- d.x, d.y ∈ {-1, 0, +1}
+end
+```