feat: add humanize() for human-readable relative time formatting

[ZialeHub]

Summary

timeflow currently provides precise formatting (via strftime-style format strings) and exact elapsed durations. A common complementary need — especially in UIs, logs, notifications, and reports — is relative human-readable formatting: expressing a point in time in natural language relative to the current moment or another reference point.

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Proposed API

humanize() — relative to now

datetime.humanize()? -> String

// Examples (relative to current system time):
// "just now"          (< 30 seconds ago)
// "2 minutes ago"
// "an hour ago"
// "3 days ago"
// "last month"
// "2 years ago"
// "in 5 minutes"      (future)
// "in 3 days"
// "in 2 months"

humanize_relative_to(&reference) — relative to a given point

datetime.humanize_relative_to(&other_datetime)? -> String
date.humanize_relative_to(&other_date)? -> String

This is useful for displaying times relative to a known event (e.g. "3 hours after the deployment", "1 week before the deadline") without being tied to the current wall clock.

age_in_years(&reference) — for birthdate arithmetic

let birthdate = Date::new(1990, 6, 15)?;
let today = Date::now()?;
let age = birthdate.age_in_years(&today)?;  // e.g. 34

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Threshold Table

Suggested rounding thresholds (inspired by moment.js and timeago.rs):

Elapsed	Output
< 30s	"just now" / "in a moment"
30s – 90s	"a minute ago" / "in a minute"
90s – 45min	"X minutes ago"
45min – 90min	"an hour ago" / "in an hour"
90min – 22h	"X hours ago"
22h – 36h	"a day ago" / "tomorrow" / "yesterday"
36h – 26 days	"X days ago"
26d – 46d	"a month ago" / "next month"
46d – 11 months	"X months ago"
11m – 18m	"a year ago" / "next year"
> 18 months	"X years ago"

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Implementation Notes

• humanize() calls humanize_relative_to(&Self::now()?) internally.
• The method should be available on Date, Time, and DateTime.
• For Date::humanize(), time-of-day granularity is not available — the threshold table should stop at "days" for Date.
• For Time::humanize(), only intra-day units (hours, minutes, seconds) are meaningful.
• All output strings should be in English for v1. Localisation is explicitly out of scope for this issue.
• humanize does not need to be configurable in v1 — the threshold table above can be hardcoded. A HumanizeOptions builder could be added later.
• This feature has no new dependencies — it uses only Self::now() and elapsed() which already exist in timeflow.

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Acceptance Criteria

• humanize() implemented on Date, Time, and DateTime
• humanize_relative_to(&reference) implemented on all three types
• age_in_years(&reference) implemented on Date
• Future times produce "in X ..." phrasing; past times produce "X ... ago"
• Threshold table is applied correctly (tested with specific elapsed durations)
• Date::humanize() does not produce sub-day output
• Time::humanize() does not produce multi-day output
• Unit tests covering each threshold bucket for past and future
• No new crate dependencies introduced
• Doc comments with examples showing past and future output