docs: add input-and-placement OpenSpec change and update AGENTS.md

Plan input handling and pattern placement before Game of Life mechanics:
keyboard bindings config, mouse placement with preview/rotation,
HiDPI scaling, and cell grid. Update AGENTS.md project notes.

openspec/changes/input-and-placement/specs/display-scaling/spec.md

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+# Spec Delta
+
+## Purpose
+
+Keeps the application rendering crisp on displays with integer or fractional scaling by converting between logical and physical coordinates.
+
+## ADDED Requirements
+
+### Requirement: Fractional scale is applied when available
+The system SHALL use the compositor-provided fractional preferred scale for rendering when the fractional scale protocol is available.
+
+#### Scenario: Fractional display
+- **WHEN** the compositor sends a preferred scale of 1.5
+- **THEN** the render buffer is created at 1.5 times the logical size
+- **AND** the buffer is displayed at the logical size
+
+### Requirement: Integer scale is used as fallback
+The system SHALL use the output's integer scale when fractional scale is unavailable.
+
+#### Scenario: No fractional protocol
+- **WHEN** the compositor does not support fractional scaling
+- **THEN** the render buffer uses the output integer scale
+- **AND** the buffer is displayed at the logical size
+
+### Requirement: Input coordinates remain logical
+The system SHALL report mouse positions and grid cells in logical coordinates regardless of the active scale.
+
+#### Scenario: Scaled click
+- **WHEN** the user clicks on a scaled display
+- **THEN** the selected grid cell matches the visual location of the cursor
+
+### Requirement: Rendering uses physical resolution
+The system SHALL render all content at physical resolution so that lines and cells stay sharp at the active scale.
+
+#### Scenario: Sharp cells
+- **WHEN** the application runs on a 2x scaled display
+- **THEN** cells are drawn with 2 physical pixels per logical pixel
+
+### Requirement: Scale changes trigger re-render
+The system SHALL recreate buffers and re-render when the active scale changes.
+
+#### Scenario: Scale change
+- **WHEN** the compositor changes the preferred scale
+- **THEN** the application updates its buffers
+- **AND** subsequent frames use the new scale