docs: create roadmap (6 phases)

.planning/STATE.md

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+# Project State
+
+## Project Reference
+
+See: .planning/PROJECT.md (updated 2026-02-24)
+
+**Core value:** Archive impossible to unpack without knowing the format -- standard tools (7z, tar, unzip, binwalk) cannot recognize or extract contents
+**Current focus:** Phase 1: Format Specification
+
+## Current Position
+
+Phase: 1 of 6 (Format Specification)
+Plan: 0 of 1 in current phase
+Status: Ready to plan
+Last activity: 2026-02-24 -- Roadmap created
+
+Progress: [░░░░░░░░░░] 0%
+
+## Performance Metrics
+
+**Velocity:**
+- Total plans completed: 0
+- Average duration: -
+- Total execution time: 0 hours
+
+**By Phase:**
+
+| Phase | Plans | Total | Avg/Plan |
+|-------|-------|-------|----------|
+| - | - | - | - |
+
+**Recent Trend:**
+- Last 5 plans: -
+- Trend: -
+
+*Updated after each plan completion*
+
+## Accumulated Context
+
+### Decisions
+
+Decisions are logged in PROJECT.md Key Decisions table.
+Recent decisions affecting current work:
+
+- Roadmap: Format spec must precede all implementation (all three decoders build against same spec)
+- Roadmap: Obfuscation (XOR headers, encrypted TOC, decoy padding) deferred to Phase 6 after all decoders work without it
+
+### Pending Todos
+
+None yet.
+
+### Blockers/Concerns
+
+- Open question: Does target busybox have `openssl enc -aes-256-cbc` with `-K`/`-iv` flags? (affects Phase 5)
+- Open question: Is `xxd` available in target busybox? Fallback: `od` (affects Phase 5)
+- Open question: Should HMAC use same key as AES or derived subkey? (must resolve in Phase 1 spec)
+
+## Session Continuity
+
+Last session: 2026-02-24
+Stopped at: Roadmap created, ready to plan Phase 1
+Resume file: None