feat: implement keyboard and mouse input, pattern placement, and HiDPI scaling

Add configurable key bindings (TOML), modifier-aware dispatch, mouse-driven
pattern placement with preview and rotation, cell grid storage/rendering,
and fractional/integer HiDPI scaling. Archive the input-and-placement change
and sync its specs to main specs.

Tests: Hashtable, Config, Grid, Pattern (4/4 pass).

openspec/specs/pattern-placement/spec.md

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+# pattern-placement Specification
+
+## Purpose
+Lets users select a Game of Life pattern, preview it under the mouse cursor, rotate it, and place copies on the grid with a click.
+
+## Requirements
+
+### Requirement: Selecting a pattern arms a placement tool
+The system SHALL activate a placement tool for the selected pattern when a `select:<pattern>` action is triggered, and show a preview following the cursor.
+
+#### Scenario: Tool activation
+- **WHEN** the user triggers `select:glider`
+- **THEN** a glider preview appears at the current cursor position
+- **AND** the placement tool remains active
+
+### Requirement: Preview follows cursor motion
+The system SHALL update the preview position to track the current mouse cursor cell while a tool is active.
+
+#### Scenario: Mouse moves
+- **WHEN** the user moves the mouse while a tool is active
+- **THEN** the preview moves to the cell under the cursor
+
+### Requirement: Tool can be rotated in 90-degree steps
+The system SHALL rotate the active preview by 90 degrees each time the rotate action is triggered.
+
+#### Scenario: Rotate preview
+- **WHEN** the user triggers the rotate action while a tool is active
+- **THEN** the preview rotates 90 degrees clockwise
+- **AND** remains at the same anchor cell
+
+### Requirement: Left click places the pattern when valid
+The system SHALL write the active pattern's alive cells into the grid at the preview position when the user left-clicks and the pattern fits entirely within the grid.
+
+#### Scenario: Valid placement
+- **WHEN** the user left-clicks while a tool is active and the preview fits in the grid
+- **THEN** the pattern's cells become alive in the grid
+- **AND** the tool remains active for further placement
+
+### Requirement: Invalid placement is rejected
+The system SHALL not modify the grid when a left click occurs while the preview extends outside the grid.
+
+#### Scenario: Out of bounds
+- **WHEN** the user left-clicks while the preview extends outside the grid
+- **THEN** no cells are changed
+- **AND** the tool remains active
+
+### Requirement: Escape cancels the active tool
+The system SHALL deactivate the placement tool and remove the preview when the cancel action is triggered.
+
+#### Scenario: Cancel placement
+- **WHEN** the user triggers the cancel action while a tool is active
+- **THEN** the preview disappears
+- **AND** no further clicks place a pattern until a tool is selected again
+
+### Requirement: Click without active tool does nothing
+The system SHALL ignore left clicks when no placement tool is active.
+
+#### Scenario: No tool
+- **WHEN** the user left-clicks while no tool is active
+- **THEN** the grid is unchanged