Support rendering multiple source files in a single session

[airvzxf]

Summary

mdfried currently accepts a single positional [SOURCE]. A shell
glob like mdfried **/PRO*.md that expands to multiple paths is
rejected with clap's generic error: unexpected argument ... found,
and the user has to either quote the glob or run mdfried once per
file.

This issue asks for first-class multi-source support: pass several
files on the command line, render them in sequence, and place a thin
separator between them so the boundary is visible.

Reproduction

touch a.md b.md
mdfried a.md b.md
# error: unexpected argument 'b.md' found

or, in a tree with multiple matches:

mdfried **/PRO*.md
# error: unexpected argument 'firefox-extension-efficient-tab-manager/PROPOSAL.md' found

Expected

mdfried a.md b.md

should render a.md and b.md in sequence, with a single muted
separator between them that includes the next file's name, e.g.

\u2500\u2500\u2500\u2500\u2500 File: a.md \u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500
   1 \u2502 # a
   2 \u2502 body of a
\u2500\u2500\u2500\u2500\u2500 File: b.md \u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500
   1 \u2502 # b
   2 \u2502 body of b

The path in the separator should be exactly what the user typed on the
command line (so a relative glob like firefox-extension-.../PROPOSAL.md
stays relative and scannable), not an absolute / canonicalized path.
Content of each file is unchanged.

Why

This matches the ergonomics of bat, which is the most-cited
alternative when users reach for a glob in the first place. It also
removes the friction of having to wrap shell loops around mdfried
to view a directory of proposals, READMEs, or notes.

Scope (v1)

• File paths only.
• Each file's relative ![](img.png) resolves against that file's
  own basepath (so images in subdir/a.md resolve to
  subdir/img.png, not to the first file's basepath).
• A separator is rendered before every file (including the first),
  matching bat.
• --watch is rejected when more than one source is passed, with a
  friendly usage error (--watch is only supported with a single source).
• URLs (http://, https://), 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.
• No new dependencies.

Out of scope (v2 follow-ups)

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

Notes

This issue is independent of #178 / #179 (the friendlier error
message for the current rejection). When this lands, the friendly
error in #179 becomes unnecessary because the rejection no longer
happens.

[benjajaja]

Too complex for little benefit, mdfried is inherently a single document viewer.

You could just concat your files beforehand, or generally work it out with some scripting.