Define local progress save model

[hideyukiMORI]

Problem or opportunity

KeyQuest needs a durable local progress model before XP, streaks, achievements, and daily sessions can grow safely.

Desired outcome

Define and document the save-file shape for session history, XP, streaks, settings, locale, skill tracks, and unlocked rewards.

Acceptance criteria

• Save data fields are documented.
• The model supports 90-day progress, streaks, achievements, weak-key stats, and locale settings.
• Future migrations are considered.
• Follow-up implementation tasks are clear.

Verification plan

Review docs and add tests when implementation starts.

[hideyukiMORI]

Additional persistence direction:

• Normal mode should avoid plain JSON saves. Prefer canonical JSON -> compression -> encoded payload with integrity check such as HMAC/checksum.
• This is tamper friction, not real security against a determined local attacker.
• --dev / -dev should write readable JSON for debugging.
• Dev mode must show a visible DEV MODE marker in the terminal.
• Dev-mode clears, achievements, or endings should be marked as development results and feel intentionally less prestigious than normal-mode clears.
• See docs/SAVE_DATA.md for the current policy.