[WilliamRagstad]

Preserve the current room, inventory, flags, one-shot hotspots, active dialogue node, dialogue history and ending across save/load. A player can save during the keeper's instructions, start a fresh game, reload and finish the adventure without losing progress.

Saves use a versioned JSON envelope and a SHA-256 scene fingerprint. Loading validates references and state consistency before replacing live state. Saving syncs a temporary file and atomically replaces the slot. Incompatible or corrupt saves report field/path diagnostics and leave the running game unchanged. Scene content changes intentionally invalidate v1 saves; migrations are not implemented.

Adds F5/F9 and mouse save/load controls, an interactive terminal player, a self-checking walkthrough with three disk checkpoints, save-format documentation, a captured transcript and native screenshots in docs/walkthrough.md.

Validation:

• cargo test --locked: all 18 tests passed, including fresh-instance restoration after every gameplay step and invalid-save/I/O cases.
• cargo clippy --locked --all-targets --all-features -- -D warnings, formatting, and the native desktop build passed.
• cargo run --locked --example console -- --walkthrough: reached and restored the safe_harbor ending.
• Native smoke check under Xvfb/Mesa Vulkan: mouse pickup/dialogue, keyboard navigation, blocked puzzle actions, F5/F9 and mouse Save/New/Load, item consumption, ending restoration, and the 960×760 minimum window.

Play: cargo run --locked --features desktop --example last_light.

Third of three stacked changes; depends on #2 (which depends on #1) and targets its branch. Retarget to main after the preceding changes merge. Fixture assets are original CC0 content with Bevy font attribution.