feat(custom-buffer-file): make C-; b p dispatch by major-mode, with tests

Old behavior: `C-; b p' called `cj/copy-path-to-buffer-file-as-kill',
which only worked in file-visiting buffers and errored otherwise.
That meant the most useful "give me a clickable handle on this
buffer" key did nothing in eww, elfeed, dired (file-at-point ≠
buffer's default-directory), and other browsing-shaped modes.

Replace with a `major-mode'-aware dispatch:

- `cj/buffer-source-functions' alist maps major-mode → thunk
  returning a string (or nil to fall through).
- `cj/copy-buffer-source-as-kill' looks up the current mode,
  calls the thunk, falls back to `buffer-file-name', errors only
  when both yield nil.
- `cj/copy-path-to-buffer-file-as-kill' kept as a `defalias' for
  backwards compat (the old name is referenced in adjacent tests).

First-batch dispatches:
- eww-mode         -> (eww-current-url)
- elfeed-show-mode -> (elfeed-entry-link elfeed-show-entry)
- dired-mode       -> (dired-get-filename nil t)
- dirvish-mode     -> same
- doc-view / pdf-view: covered by the buffer-file-name fallback
  (they already set buffer-file-name correctly).

10 new ERT tests cover the dispatch paths, the
buffer-file-name fallback, the user-error on nil source, the alias
target, and the `C-; b p' keymap entry.

which-key label flipped from "copy file path" to "copy buffer
source" to match.

Deferred to a follow-up task: mu4e-view-mode, org-mode at a
heading, help-mode, Info-mode, magit-log/commit/status, xref/grep/
compilation, image-mode, archive-mode -- each needs a format
decision before implementation.

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