Describe shipped presets and looks in the manual and FR-DEV-6

The presets sheet now lists the shipped collection beside the
photographer's own, a copy under a shipped name overrides it, and
shipped and imported presets are looks that leave a photograph's own
corrections alone. The manual says so where it introduces the sheet,
and FR-DEV-6 states the rule. The sheet's screenshot (media/presets.png)
predates the sections and needs re-recording.

docs/dev/requirements.md

@@ -502,6 +502,13 @@ so history survives a restart. Named snapshots of an edit state.
 **FR-DEV-6 — Presets.** Save, apply, and manage named presets covering a subset of the edit
 graph. Copy/paste settings between images. Batch-apply to a selection.
 
+A preset may name a film stock, which travels with the film node's own parameters. The
+application ships a read-only collection of presets, updated with each release and never
+written into the user's library; a user preset saved under a shipped name overrides it
+until deleted or renamed. Shipped and imported presets change only the operations they
+name, so a look applied to a corrected photograph keeps the correction; a copy or a saved
+edit replaces everything in scope.
+
 **FR-DEV-7 — Before/after.** Compare current edit state against the unedited original or against
 a chosen history state.