feat(P.3.6): lib-core.selection v0.1.0 — initial release

Handle-Registry + Click/Drag-Box State-Machine + Selection-Set-API +
alpha-tinted render-helpers. Release-edge detection internal via
prev_click_down (no was_action_released in lib-core.input).

See: meta/docs/superpowers/specs/2026-05-14-p3-6-lib-selection-design.md

README.md

@@ -0,0 +1,45 @@
+# lib-core.selection — v0.1.0
+
+Click-Select + Drag-Box-Select + Selection-Set-State.
+
+## Quick Setup
+
+```lua
+local selection = require("lib-core.selection")
+local input     = require("lib-core.input")
+
+input.bind("lmb",        { "mouse_left" })
+input.bind("mod_add",    { "shift" })
+input.bind("mod_toggle", { "ctrl" })
+
+selection.bind_action("lmb")
+selection.bind_modifier_add("mod_add")
+selection.bind_modifier_toggle("mod_toggle")
+
+local handle = selection.register(function()
+    return { x = entity.x, y = entity.y, w = entity.w, h = entity.h }
+end)
+
+-- per-frame:
+function update(ctx, dt)  selection.update(dt)  end
+function render(ctx)
+    camera.begin()
+        -- entities…
+        selection.render_highlights()    -- world-space
+    camera.finish()
+    selection.render_drag_box()          -- screen-space
+end
+```
+
+## API
+
+- Registry: `register(aabb_fn) → handle`, `unregister(h)`, `count_registered()`
+- Set-Query: `list()`, `count()`, `contains(h)`, `is_empty()`
+- Set-Mutation: `add(h)`, `remove(h)`, `toggle(h)`, `clear()`
+- Bindings: `bind_action(name)`, `bind_modifier_add(name)`, `bind_modifier_toggle(name)`
+- State-Config: `set_drag_threshold(px)` / `drag_threshold()`, `set_enabled(bool)` / `enabled()`
+- State-Query: `drag_state() → "idle"|"ambiguous"|"dragging"`, `box_rect() → {x,y,w,h}|nil`
+- Visual: `set_drag_box_color(r,g,b,a)` / `drag_box_color()`, `set_highlight_color(r,g,b,a)` / `highlight_color()`
+- Per-Frame: `update(dt)`, `render_drag_box()`, `render_highlights()`
+
+See `meta/docs/superpowers/specs/2026-05-14-p3-6-lib-selection-design.md` for full design.