feat(cli): render multiple source files in one session

[airvzxf]

Closes #180

What

mdfried now accepts more than one positional [SOURCE] and
renders them in sequence, separated by a thin bat-style rule line
that includes the next file's name. mdfried a.md b.md c.md works
end-to-end.

mdfried a.md b.md

renders a.md, then a single muted separator line (\u2500\u2500\u2500\u2500\u2500 File: b.md \u2500\u2500...), then b.md. Markdown content inside each
file is not modified.

Why

A shell glob like mdfried **/PRO*.md expands to multiple paths and
currently fails with clap's generic unexpected argument ... found.
The user has to either quote the glob or loop over mdfried
manually. First-class multi-source support makes the common case
("show me all PROPOSAL.md files") one command, matching bat's
ergonomics.

How

• New DocumentSource::MultiFile { entries: Vec<MultiFileEntry> }
  variant carries the pre-loaded text per file, so the worker can
  parse each in order without re-reading the filesystem.
• New SectionContent::FileSeparator { filename } variant is what
  the view layer renders as a single muted line, using the same
  \u2500 character bat uses.
• New Cmd::ParseMulti command asks the worker to parse a list of
  (text, basepath) pairs. parse_one_file and drain_image_cache
  are extracted from the existing Cmd::Parse flow so both arms
  share the parsing and image-cache code.
• Each section produced by parse_one_file in multi-file mode
  carries the current file's basepath through SectionEvent::Image
  to image_section, so relative ![](img.png) references resolve
  against the correct file. Single-file behaviour is unchanged because
  image_section still falls back to document_source.read() when
  no basepath override is given.
• Model::parse_multi sends Cmd::ParseMulti to the worker.
• The CLI validates the source list before any I/O: a friendly usage
  error is returned for --watch + multiple sources, multiple -,
  mixed stdin + files, and URLs alongside file paths.

v1 scope (intentional)

• File paths only. URLs, github:owner/repo, and - (stdin)
  remain single-source for v1; mixing them with file paths fails
  with a friendly usage error pointing at the limitation.
• A separator is rendered before every file (including the first),
  matching bat.
• --watch rejects more than one source.
• No new dependencies.

A new unit test multi_file_renders_with_file_separator builds a
Model with a MultiFile source, calls parse_multi for two
trivial markdown files, and asserts the resulting sections include a
FileSeparator for each file with the user-supplied path verbatim,
and that the rendered buffer contains the bat-style rule character
around each filename. The full test suite (cargo test --bin mdfried)
passes (34/34). cargo clippy --all-targets -- -D warnings and
cargo doc --no-deps are clean. cargo build --release works.

Test plan for the reviewer

# Reject the failure mode
mdfried a.md b.md c.md      # works, renders in order
mdfried **/*.md             # works, the original report

# Rejections (should print a friendly usage error and exit 2):
mdfried a.md b.md --watch
mdfried a.md https://example.com/foo.md
mdfried - a.md
mdfried a.md -

Out of scope (v2)

• URL / github / stdin mixed with file paths.
• Multi-file --watch.
• A multi-file :back history.
• Per-file deep_fry overrides.

Independent of #178 / #179

This PR does not interact with #178 or #179. When this lands, the
friendly error in #179 becomes unnecessary because the rejection no
longer happens for the file-path case.

[Copilot]

Pull request overview

Adds first-class multi-source rendering to mdfried, allowing multiple file paths to be rendered sequentially in one session with a bat-style separator line, while preserving per-file relative image resolution.

Changes:

• Extend the source/model/worker pipeline to support DocumentSource::MultiFile and Cmd::ParseMulti, parsing multiple in-memory file entries in order.
• Introduce SectionContent::FileSeparator and render it as a muted bat-style rule line in the view layer.
• Thread per-file basepaths through image post-processing so relative image links resolve against the correct file in multi-file mode.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/main.rs	CLI now accepts multiple positional sources and performs upfront validation before dispatching multi-file parsing.
src/sources.rs	Adds MultiFileEntry and DocumentSource::MultiFile to carry per-file preloaded text/basepaths.
src/model.rs	Adds Model::parse_multi and blocks link navigation in multi-file mode.
src/worker.rs	Adds Cmd::ParseMulti, refactors shared parse logic (parse_one_file, drain_image_cache), and emits FileSeparator sections.
src/worker/sections.rs	Adds iterator support for non-zero starting section IDs to keep section IDs unique across files.
src/view.rs	Renders SectionContent::FileSeparator as a single muted rule line containing the filename.
src/document.rs	Adds SectionContent::FileSeparator and adds a basepath override to image_section to support per-file relative images.

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[benjajaja]

Not good, and too complex for little benefit even if it wasn't AI generated. It would be a lot better if you would discuss changes before opening these big PRs.