feat(promql): typed duration builders (seconds/minutes/.../duration)

[pyramation]

Summary

Adds typed duration builders to promql-ast so callers never hand-assemble PromQL duration syntax with string interpolation (e.g. `${n}s`). The syntax now lives in the package, which is the whole point of the AST.

import { rate, range, metric, seconds, minutes, duration } from 'promql-ast';

range(metric('x'), seconds(60));       // x[60s]
range(metric('x'), minutes(5));        // x[5m]
duration({ h: 1, m: 30 });             // '1h30m'

New exports: seconds, minutes, hours, days, weeks, years, milliseconds, and the composite duration({ y,w,d,h,m,s,ms }) (throws on an empty spec). A branded Duration type is added; range / subquery / offset now take Duration instead of string.

Backwards compatible: Duration's brand is optional, so existing call sites passing plain string literals (range(sel, '5m')) still typecheck.

Version bump 0.2.1 → 0.3.0.

Tests

+3 tests (duration composition, empty-spec rejection, range(metric, seconds(60)) → x[60s]). Full suite 51 green; tsc clean; makage build (CJS+ESM) green.

Follow-up

constructive-db#2275's collector will use seconds() for its rate window instead of `${rateWindowSeconds}s` once this is published.

Link to Devin session: https://app.devin.ai/sessions/16409487e97d44828dfb46277767604b
Requested by: @pyramation

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