docs(mcap): split legacy single-camera layout reference

Move the legacy /camera/* contract into its own reference page so the main MCAP layout doc can stay focused on current bundled and copy-layout behavior.

Document the compatibility model explicitly: legacy single-camera is operationally equivalent to one-camera copy when the effective camera label is treated as the literal `camera`.

Update the mcap_rgbd_example helper and recipe docs to accept legacy /camera/* inputs under that compatibility rule instead of rejecting them.

docs/mcap_recipes.md

@@ -4,12 +4,12 @@ This guide is the simple, non-GUI path for inspecting RGB+depth MCAP files.
 
 Use it when you want to:
 
-- confirm whether an MCAP is bundled or `copy`
+- confirm whether an MCAP is bundled, `copy`, or legacy `/camera/*`
 - inspect camera labels, message counts, and timestamp ranges
 - export one RGB frame and one decoded depth sample as a concrete example
 - understand how `/bundle` changes the meaning of timestamps and sample grouping
 
-For the full layout contract, see [mcap_layout.md](./mcap_layout.md).
+For the current bundled/copy layout contract, see [mcap_layout.md](./mcap_layout.md). The older `/camera/*` wire shape is documented separately in [mcap_legacy_single_camera_layout.md](./mcap_legacy_single_camera_layout.md).
 
 ## Quick Summary
 
@@ -41,14 +41,19 @@ uv sync --extra viewer
 
 ## The Practical Cases
 
-For this helper, there are really two supported layouts:
+For this helper, there are really two operational cases:
 
 - `bundled`: multiple namespaced camera topics plus `/bundle`
-- `copy`: namespaced camera topics with no `/bundle`
+- single-camera stream with no `/bundle`
 
-Current single-source `zed_svo_to_mcap` output uses the one-camera `copy` shape by default, so even a one-camera file still looks like namespaced `/{label}/*` topics with no `/bundle`.
+That second case can appear in two wire shapes:
 
-Legacy `/camera/*` MCAP files are intentionally out of scope for this helper. Regenerate them into `copy` layout if you still have old files around.
+- `copy`: namespaced topics such as `/zed4/video`
+- legacy single-camera: `/camera/video`
+
+Current single-source `zed_svo_to_mcap` output uses the one-camera `copy` shape by default, so even a one-camera file usually looks like namespaced `/{label}/*` topics with no `/bundle`.
+
+The helper treats legacy `/camera/*` as compatible with `copy` by using the implicit camera label `camera`.
 
 ## Recipe: Summarize One MCAP
 
@@ -70,7 +75,7 @@ What the summary prints:
 
 This is the fastest way to answer:
 
-- “is this file bundled or copy?”
+- “is this file bundled, copy, or legacy single-camera?”
 - “which camera labels are inside?”
 - “do video and depth counts match?”
 - “what timestamp range does each camera cover?”
@@ -83,7 +88,7 @@ uv run python scripts/mcap_rgbd_example.py export-sample \
     --output-dir /tmp/mcap_sample
 ```
 
-For multi-camera or one-camera `copy` files, choose the camera explicitly when needed:
+For multi-camera or namespaced one-camera files, choose the camera explicitly when needed:
 
 ```bash
 uv run python scripts/mcap_rgbd_example.py export-sample \
@@ -104,25 +109,26 @@ Outputs:
 
 That means:
 
-- `copy`: sample `N` is `/zedN/video[N]` + `/zedN/depth[N]`
+- legacy `/camera/*`: sample `N` is `/camera/video[N]` + `/camera/depth[N]`
+- `copy`: sample `N` is `/{label}/video[N]` + `/{label}/depth[N]`
 - `bundled`: sample `N` is the `N`th present sample for that camera, not bundle index `N`
 
 In bundled files, `sample_metadata.json` also records the matched `/bundle` member metadata for the selected camera sample.
 
-## Recipe: Understand Bundled vs Copy Timing
+## Recipe: Understand Bundled vs Non-Bundled Timing
 
 Bundled files intentionally separate bundle time from camera sample time:
 
 - `/bundle.timestamp` is the nominal common-timeline bundle timestamp
 - `/zedN/video` and `/zedN/depth` keep the original per-camera sample timestamps
 
-Copy files do not have bundle time at all:
+Copy and legacy single-camera files do not have bundle time at all:
 
 - there is no `/bundle`
 - each camera topic keeps its own original cadence and timestamps
 
 If you care about grouping, use `/bundle` in bundled files.
-For `copy`, treat each camera stream independently.
+For `copy` and legacy single-camera files, treat each camera stream independently.
 
 ## Recipe: Inspect `/bundle` In Python
 
@@ -169,4 +175,5 @@ with path.open("rb") as stream:
 This is the important mental model:
 
 - `bundled`: follow `/bundle` for grouping
-- `copy`: treat each camera as an independent stream
+- `copy`: treat each namespaced camera as an independent stream
+- legacy `/camera/*`: same model as one-camera `copy`, with the implicit label `camera`