docs(adr): 0001 observation store in SQLite; glossary terms for store entities

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+# Fenris
+
+Fenris observes an NVMe drive’s real-world use and translates that history into an understandable endurance outlook.
+
+## Language
+
+**Observation history**:
+The persisted record of drive activity gathered while Fenris monitoring is enabled, retained across restarts and reboots.
+_Avoid_: Calibration data, temporary history
+
+**Observed usage habit**:
+The pattern of active, idle, and powered-off hours represented by the observation history, with recent sustained behavior carrying more relevance than distant behavior.
+_Avoid_: Current usage, benchmark workload
+
+**Usage-adjusted theoretical lifespan**:
+The theoretical time until the drive’s write endurance is exhausted if its observed usage habit continues; it is an endurance projection, not a predicted hardware-failure date.
+_Avoid_: Future life, actual lifespan, failure date
+
+**Projection confidence**:
+The degree to which the observation history is sufficiently long, complete, and stable to support the usage-adjusted theoretical lifespan.
+_Avoid_: Accuracy percentage, certainty
+
+**Monitoring period**:
+A span during which Fenris monitoring is enabled; powered-off time remains part of the usage habit, while deliberately disabled time does not.
+_Avoid_: Daemon uptime, calibration window
+
+**Observation store**:
+The single SQLite database at `/var/lib/fenris/observations.db` that persists the observation history, monitoring periods, hour observations, day aggregates, and endurance baseline.
+_Avoid_: Data directory, history.jsonl, the database (generic)
+
+**Hour observation**:
+One row per UTC hour in the observation store, recording that hour's usage-habit split into active, idle, powered-off, and unknown seconds, plus write/read deltas, thermal evidence, and coverage.
+_Avoid_: Hourly record, hourly.jsonl entry
+
+**Day aggregate**:
+One row per UTC day derived from hour observations; the grain at which usage-habit evidence is judged.
+_Avoid_: Daily summary, daily stats
+
+**Controller segment**:
+A span of observation history within which the drive's controller identity is unchanged and counters are monotonic; write deltas are never computed across a segment boundary.
+_Avoid_: Counter reset handling, drive swap detection
+
+**Endurance baseline**:
+The write-endurance value a projection consumes: a verified rated-TBW override stored with provenance when one exists, otherwise a coarse implied baseline derived from vendor wear and labeled as such.
+_Avoid_: TBW value, failure threshold, max writes