feat(core): add pattern algebra transforms on HapticPattern

[l2hyunwoo]

Summary

Compiled patterns should be manipulable as plain values. This adds pure transforms on HapticPattern, each returning a new pattern:

fun HapticPattern.scaleIntensity(factor: Float): HapticPattern
fun HapticPattern.timeStretch(factor: Float): HapticPattern
fun HapticPattern.reversed(): HapticPattern
infix fun HapticPattern.then(other: HapticPattern): HapticPattern
operator fun HapticPattern.plus(other: HapticPattern): HapticPattern
fun HapticPattern.repeated(times: Int): HapticPattern

HapticPattern.events is already public, so these are extension functions over the event list. No executor or platform code is involved.

Design decisions

• scaleIntensity maps every intensity to HapticIntensity.Custom(value * factor). The Custom constructor already clamps to [0, 1] (HapticIntensity.kt:61), so no extra guard is needed. Predefined levels are downgraded to Custom, since a scaled STRONG has no predefined equivalent. Negative factors are rejected with require.
• timeStretch scales event edges, not durations: newStart = round(start * f), newEnd = round((start + duration) * f), newDuration = newEnd - newStart. Rounding start and duration independently would let adjacent event boundaries drift apart.
• reversed() mirrors each event on the pattern span: newStart = span - (start + duration). Transforms may produce overlapping events and do not resolve them; playback serializes overlaps through mergeToSerial (highest intensity wins). That is why this work depends on feat(core): serial-timeline waveform pipeline (overlap, fall-ramp, handle lifecycle) #85.
• repeated is defined on top of then, so repeated(n) equals folding then n times by construction.
• rawSpanMs() (executor/RawSpanMs.kt:27) and PatternElement.totalDurationMs (element/PatternElement.kt:55) compute the same max end time over events. Both delegate to a single internal HapticPattern.spanMs() after this change. The tree-sum totalDurationMs overrides on SequenceElement/RepeatElement are sequential-scheduling math, not span, and stay untouched.

Acceptance criteria

Property tests (kotest-property, same idiom as InsertFallRampsTest):

• p.reversed().reversed() == p
• p.timeStretch(1f) == p; p.timeStretch(a).timeStretch(b) within events.size ms of p.timeStretch(a * b)
• span(p.timeStretch(f)) equals round(span(p) * f) within rounding tolerance
• intensity stays in [0, 1] after any scaleIntensity
• then is associative; the empty pattern is its identity; span(a then b) == span(a) + span(b)
• p.repeated(1) == p; p.repeated(0) is empty; event count multiplies by times

One top-level declaration per file, apiDump regenerated.

Depends on #85.