mboxShell

Traceable merges, and docs that match the tool

• merge --source-header now records the mailbox name you know. Apple Mail exports a mailbox as a folder "Inbox.mbox" holding a file called literally "mbox", so every message got stamped X-Mbox-Source: mbox — useless for the one thing the header exists for. Mailboxes that would share a name are told apart by their folder.
• The docs described a merge --dedup flag that does not exist: deduplication is on by default and the real flag is --no-dedup. Both READMEs and both manuals are corrected, and --source-header — which shipped undocumented in 0.6.1 — is now covered.
• The merge summary line "Tagged with source" was hardcoded in English while every other line was translated — it now goes through the catalogue too. 7 new tests (202 in total).

Trace every merged message back to its mailbox

• New merge --source-header option: every merged message gets an X-Mbox-Source: <origin file> header as its first header, so a combined archive stays traceable. It reuses the message's own line endings, preserves a leading BOM, and sanitizes the file name so a crafted one can't inject extra headers.

Interactive performance on very large mailboxes

• The message view no longer re-wraps the whole body on every frame — styled lines are cached and reused.
• Incremental search is debounced into a single pass that runs after typing settles (~150 ms).
• Selecting a message no longer deep-copies its decoded body; bodies are shared by reference for cheaper selection.