Standardize Reader/Writer public API naming and return types

[StephanMeijer]

Problem

Every reader and writer module exposes a different function name and return shape, making the library harder to learn and use consistently.

Module	Entry function	Returns
DOCX.Reader	open!/1 → convert!/1 → close!/1	Bare struct (raises)
Tiptap.Reader	convert/1	{:ok, spec} | {:error, ...}
HTML.Writer	convert/1	String.t() (bare, no error path)
EPUB.Writer	convert!/1 / convert!/2	Mixed (see #19)
Tiptap.Writer	convert/1	{:ok, element} | {:error, ...}
BlockNote.Writer	write/1	{:ok, content} | {:error, ...}
Validation.Writer	validate/1	[Finding.t()] (bare list)

Specific inconsistencies

1. BlockNote.Writer.write/1 uses write while all other writers use convert
2. HTML.Writer.convert/1 returns a bare string with no error tuple — no way to handle conversion failures
3. DOCX.Reader requires a 3-step open/convert/close lifecycle; Tiptap.Reader is a single convert/1 call
4. Validation.Writer — the module is named "Writer" but the public function is validate/1, which is semantically correct but structurally confusing given the module hierarchy

Proposal

Standardize on:

• convert/1 → {:ok, result} | {:error, reason} (all readers and writers)
• convert!/1 → result or raises (convenience bang variant)
• DOCX Reader: provide a single-call convert/1 that handles the open/convert/close lifecycle internally, keeping the granular API available for advanced use

Files

• lib/docspec/core/blocknote/writer.ex
• lib/docspec/core/html/writer.ex
• lib/docspec/core/epub/writer.ex
• lib/docspec/core/tiptap/reader.ex
• lib/docspec/core/tiptap/writer.ex
• lib/docspec/core/docx/reader.ex
• lib/docspec/core/validation/writer.ex

Notes

This is a breaking change. Consider whether Validation.Writer should be renamed to DocSpec.Core.Validation since it doesn't "write" output.