Support time-based -r ranges (relative and ISO)

docs/database.md

@@ -79,6 +79,7 @@ A revision number identifies a *state position between diffs*, not a diff:
 
 - **Unit specs** — any spec with a letter-prefixed bound (`l`, `s`, `v`, `r`, including mixed forms like `2:s3`) — index into the database file and resolve to a line window. Multiple unit specs intersect restrictively: only the common line range is processed.
 - **Plain-number specs** — both bounds (or the single index) are plain numbers — index into the *filtered output*: the records left after all unit specs and all `--grep` patterns have been applied. Negative values count from the end of the filtered output.
+- **Time specs** — a relative `N` + unit (`s`, `min`, `h`, `d`, `w`, e.g. `15min` or `2w`) selects the last N units ending now; a UTC ISO point (`2026-09-28`, `2026-09-28T10:30`, or compact `20260928T1030`) selects the whole natural period its precision denotes (that day, hour, minute, or second); an ISO range `start/end` is inclusive of the end's whole period, and either end may be left open. All times are UTC. Time specs match records individually by their timestamp rather than resolving to a line window, so non-monotonic timestamps simply match wherever they occur, and multiple time specs intersect like the others.
 
 Like repeated `--grep` patterns (which must all match), repeated ranges are restrictive: `-r r1:r5 -r 0:2` prints only the first two records of those in the revision window, regardless of flag order. Plain ranges truncate Python-style and print nothing when the intersection is empty; a single plain index must exist and fails with exit code 2 otherwise.