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/__init__.py

@@ -0,0 +1,130 @@
+"""
+FCPXML Writer — Generate and modify Final Cut Pro XML files.
+
+This package provides two complementary workflows for working with FCPXML:
+
+**Generation** (``FCPXMLWriter``, in :mod:`.generator`):
+    Build a new FCPXML document from Python dataclass objects (``Project``,
+    ``Timeline``, ``Clip``, ``Marker``).  Useful for creating rough cuts,
+    montage exports, and template-based projects.
+
+**Modification** (``FCPXMLModifier``, in :mod:`.modifier`):
+    Load an existing FCPXML file, apply surgical edits (markers, trims,
+    reorders, transitions, speed changes, silence removal, etc.), and save.
+    This is the primary API used by the MCP server's tool handlers.
+
+Layout
+------
+This was one 4.200-line module. It is now one module per subject, because the
+subjects barely touch each other: whoever is fixing a zoom ramp has no reason
+to scroll past subtitle layout to find it.
+
+    helpers     sanitising, scales, shared element builders
+    document    asset creation, timebases, serialisation (``write_fcpxml``)
+    validation  structural checks (``validate_fcpxml``)
+    core        ``ModifierCore``: load, indices, spine navigation, ``save``
+    <subject>   one mixin per editing subject (markers, trim, speed, …)
+    modifier    ``FCPXMLModifier`` = core + every mixin
+    generator   ``FCPXMLWriter``
+    api         one-line convenience wrappers
+
+Everything the rest of the project imported from the old module is re-exported
+here, so ``from fcpxml.writer import FCPXMLModifier`` keeps working unchanged —
+including the underscore-prefixed helpers the test suite reaches for.
+
+Architecture notes
+------------------
+- All time arithmetic uses ``TimeValue`` (rational fractions) — never floats —
+  to match FCPXML's native ``"600/2400s"`` format and avoid rounding drift.
+- The ``FCPXMLModifier`` builds three in-memory indices at init
+  (``clips``, ``resources``, ``formats``) so lookups are O(1) by ID/name.
+- Spine-based editing: clips live inside a ``<spine>`` element (the primary
+  storyline).  Connected clips attach via ``lane`` attributes on spine clips.
+  Most editing methods find the target clip in the spine, mutate it, then
+  ripple offsets on subsequent siblings.
+- ``write_fcpxml()`` handles DTD-compliant serialisation and optional
+  timebase enforcement for all output paths.
+"""
+
+from ..models import TimeValue
+from .api import add_marker_to_file, modify_fcpxml, trim_clip_in_file
+from .core import ModifierCore
+from .document import (
+    _STILL_IMAGE_EXTENSIONS,
+    _enforce_standard_timebases,
+    _ensure_video_asset,
+    write_fcpxml,
+)
+from .generator import FCPXMLWriter
+from .helpers import (
+    _ASSET_CLIP_CHILD_ORDER,
+    _CHILD_ORDER_INDEX,
+    _MAX_MARKER_NAME_LENGTH,
+    _MAX_NOTE_LENGTH,
+    CLIP_AND_AUDIO_TAGS,
+    CLIP_TAGS,
+    FCP_EFFECTS,
+    HOLD_AT_CUT_THRESHOLD,
+    SPINE_ELEMENT_TAGS,
+    START_AT_CUT_THRESHOLD,
+    _create_asset_element,
+    _dtd_insert,
+    _fmt_scale,
+    _probe_audio_info,
+    _sanitize_xml_value,
+    build_marker_element,
+    list_effects,
+)
+from .modifier import FCPXMLModifier
+from .validation import (
+    _check_asset_sources,
+    _check_child_order,
+    _check_effect_refs,
+    _check_frame_alignment,
+    _check_required_attributes,
+    _check_timebases,
+    _document_frame_duration,
+    validate_fcpxml,
+)
+
+__all__ = [
+    "FCPXMLModifier",
+    "FCPXMLWriter",
+    "ModifierCore",
+    "TimeValue",
+    "FCP_EFFECTS",
+    "CLIP_TAGS",
+    "CLIP_AND_AUDIO_TAGS",
+    "SPINE_ELEMENT_TAGS",
+    "HOLD_AT_CUT_THRESHOLD",
+    "START_AT_CUT_THRESHOLD",
+    "add_marker_to_file",
+    "build_marker_element",
+    "list_effects",
+    "modify_fcpxml",
+    "trim_clip_in_file",
+    "validate_fcpxml",
+    "write_fcpxml",
+    # Internos que o resto do projeto (e a suíte) já importava deste módulo
+    # quando ele era um arquivo só. Ficam aqui para a divisão não virar uma
+    # quebra de API disfarçada de reorganização.
+    "_ASSET_CLIP_CHILD_ORDER",
+    "_CHILD_ORDER_INDEX",
+    "_MAX_MARKER_NAME_LENGTH",
+    "_MAX_NOTE_LENGTH",
+    "_STILL_IMAGE_EXTENSIONS",
+    "_check_asset_sources",
+    "_check_child_order",
+    "_check_effect_refs",
+    "_check_frame_alignment",
+    "_check_required_attributes",
+    "_check_timebases",
+    "_create_asset_element",
+    "_document_frame_duration",
+    "_dtd_insert",
+    "_enforce_standard_timebases",
+    "_ensure_video_asset",
+    "_fmt_scale",
+    "_probe_audio_info",
+    "_sanitize_xml_value",
+]