refactor: writer.py vira pacote, um módulo por assunto

O writer tinha 4.199 linhas, das quais 3.300 numa única classe com dezoito
assuntos dentro. Achar o trecho de zoom exigia rolar por marcadores,
velocidade e legendas.

Agora é o pacote fcpxml/writer/, com um arquivo por assunto e o
FCPXMLModifier montado por composição de mixins. Mixins, e não objetos
separados, porque todas essas operações mexem no mesmo documento e nos
mesmos índices — separá-las em objetos independentes transformaria toda
chamada interna em travessia de fronteira sem nada em troca. A divisão que
importa aqui é de leitura, não de estado.

Nenhuma mudança de comportamento e nenhuma alteração nos ~50 pontos que
importam do writer: o __init__ re-exporta tudo, inclusive os nomes com
underscore que a suíte já usava.

    core      723   carga, índices, navegação na spine, save
    titles    600   títulos e legendas dinâmicas
    cut       333   dividir, cortar faixas, apagar
    speed     297   velocidade e zoom
    (+ 20 módulos menores)

Único ajuste de chamada: quatro testes faziam patch em
fcpxml.writer.subprocess, que agora mora em writer.document (ver
Engine/docs/05_EXPERIENCIAS.md #23).

Lint zerado, 1441 testes passando.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

code/fcpxml/writer/trim.py

@@ -0,0 +1,125 @@
+"""Aparar clipes e propagar o ripple pela spine.
+
+Extraído de writer.py — ver fcpxml/writer/__init__.py para o conjunto.
+"""
+
+import xml.etree.ElementTree as ET
+from typing import Optional
+
+from ..models import (
+    TimeValue,
+)
+from .helpers import SPINE_ELEMENT_TAGS
+
+
+class TrimMixin:
+    """Aparar clipes e propagar o ripple pela spine."""
+
+    # ========================================================================
+    # TRIM OPERATIONS
+    # ========================================================================
+
+    def trim_clip(
+        self,
+        clip_id: str,
+        trim_start: Optional[str] = None,
+        trim_end: Optional[str] = None,
+        ripple: bool = True
+    ) -> ET.Element:
+        """
+        Trim a clip's in-point and/or out-point.
+
+        Args:
+            clip_id: Target clip
+            trim_start: New in-point or delta ('+1s', '-10f')
+            trim_end: New out-point or delta
+            ripple: Whether to shift subsequent clips
+
+        Returns:
+            Modified clip element
+        """
+        clip = self._require_clip(clip_id)
+
+        current_start, current_duration, _ = self._get_clip_times(clip)
+
+        original_duration = current_duration
+
+        # Handle trim_start
+        if trim_start:
+            if trim_start.startswith('+') or trim_start.startswith('-'):
+                delta = self._parse_time(trim_start[1:])
+                if trim_start.startswith('-'):
+                    # Extend earlier
+                    new_start = current_start - delta
+                    new_duration = current_duration + delta
+                else:
+                    # Trim later
+                    new_start = current_start + delta
+                    new_duration = current_duration - delta
+            else:
+                new_start = self._parse_time(trim_start)
+                diff = new_start - current_start
+                new_duration = current_duration - diff
+
+            clip.set('start', new_start.to_fcpxml())
+            current_start = new_start
+            current_duration = new_duration
+
+        # Handle trim_end
+        if trim_end:
+            if trim_end.startswith('+') or trim_end.startswith('-'):
+                delta = self._parse_time(trim_end[1:])
+                if trim_end.startswith('-'):
+                    new_duration = current_duration - delta
+                else:
+                    new_duration = current_duration + delta
+            else:
+                end_point = self._parse_time(trim_end)
+                new_duration = end_point - current_start
+
+            current_duration = new_duration
+
+        if current_duration <= TimeValue.zero():
+            raise ValueError(
+                f"Trim would produce non-positive duration "
+                f"({current_duration.to_seconds():.3f}s) for clip '{clip_id}'"
+            )
+
+        clip.set('duration', current_duration.to_fcpxml())
+
+        # Ripple subsequent clips if needed
+        if ripple:
+            duration_change = current_duration - original_duration
+            if duration_change != TimeValue.zero():
+                self._ripple_after_clip(clip, duration_change)
+
+        return clip
+
+    def _ripple_from_index(
+        self, spine: ET.Element, start_index: int, delta: 'TimeValue'
+    ) -> None:
+        """Shift the offset of every spine element from *start_index* onward by *delta*.
+
+        Consolidates the ripple loops previously duplicated across
+        ``_ripple_after_clip``, ``delete_clip``, and ``insert_clip``.
+
+        Args:
+            spine: The primary storyline ``<spine>`` element.
+            start_index: First child index to adjust (inclusive).
+            delta: Signed time shift (positive = later, negative = earlier).
+        """
+        children = list(spine)
+        for child in children[start_index:]:
+            if child.tag in SPINE_ELEMENT_TAGS:
+                current_offset = self._parse_time(child.get('offset', '0s'))
+                new_offset = current_offset + delta
+                child.set('offset', new_offset.to_fcpxml())
+
+    def _ripple_after_clip(self, target_clip: ET.Element, delta: TimeValue) -> None:
+        """Shift all clips after the given clip by delta."""
+        spine = self._get_spine()
+        clip_index = self._find_clip_index(spine, target_clip)
+        if clip_index is not None:
+            self._ripple_from_index(spine, clip_index + 1, delta)
+
+    # ========================================================================