Add architecture, planning, and conventions documents

Initial documentation for the multi-peer video routing system:
- architecture.md covers graph model, transport protocol, relay design,
  and the Pi get-it-on-the-wire-first rationale
- planning.md defines module build order and directory structure
- conventions.md captures C11 code style, naming, error handling approach,
  and directory layout rules

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

planning.md

@@ -0,0 +1,70 @@
+# Planning
+
+## Approach
+
+Build the system module by module in C11. Each module is a translation unit (`.h` + `.c`) with a clearly defined API. Modules are exercised by small driver programs in `dev/` before anything depends on them. This keeps each unit independently testable and prevents architectural decisions from being made prematurely in code.
+
+The final binary is a single configurable node program. That integration work comes after the modules are solid.
+
+---
+
+## Directory Structure
+
+```
+video-setup/
+  modules/
+    common/          - shared definitions (error types, base types)
+    media_ctrl/      - Linux Media Controller API (topology, pad formats, links)
+    v4l2_ctrl/       - V4L2 camera controls (enumerate, get, set)
+  dev/
+    cli/             - exploratory CLI drivers, one per module
+    experiments/     - freeform experiments
+  tests/             - automated tests (later)
+  Makefile
+  architecture.md
+  planning.md
+  conventions.md
+```
+
+---
+
+## Module Order
+
+Modules are listed in intended build order. Each depends only on modules above it.
+
+| # | Module | Status | Notes |
+|---|---|---|---|
+| 1 | `common` | not started | Error types, base definitions — no dependencies |
+| 2 | `media_ctrl` | not started | Media Controller API — device and topology enumeration, pad format config |
+| 3 | `v4l2_ctrl` | not started | V4L2 controls — enumerate, get, set camera parameters |
+| 4 | `transport` | not started | Encapsulated transport — header encode/decode, framed TCP read/write |
+| 5 | `frame_alloc` | not started | Per-frame allocation with bookkeeping (byte budget, ref counting) |
+| 6 | `relay` | not started | Input dispatch to output queues (low-latency and completeness modes) |
+| 7 | `ingest` | not started | MJPEG frame parser (two-pass EOI state machine, opaque stream → discrete frames) |
+| 8 | `archive` | not started | Write frames to disk, control messages to JSON log |
+
+---
+
+## Dev CLI Drivers
+
+Each module gets a corresponding CLI driver in `dev/cli/` that exercises its API and serves as both an integration check and a useful development tool.
+
+| Driver | Exercises | Notes |
+|---|---|---|
+| `media_ctrl_cli` | `media_ctrl` | List media devices, show topology, configure pad formats |
+| `v4l2_ctrl_cli` | `v4l2_ctrl` | List controls, get/set values — lightweight `v4l2-ctl` equivalent |
+| `transport_cli` | `transport` | Send/receive framed messages, inspect headers |
+
+---
+
+## Deferred Decisions
+
+These are open questions tracked in `architecture.md` that do not need to be resolved before module work begins:
+
+- Graph representation format
+- Connection establishment model (push vs pull)
+- Completeness queue drop policy (oldest vs newest, per-output config)
+- Stream ID remapping across relay hops
+- Transport for relay edges (TCP / UDP / shared memory)
+- Node discovery mechanism
+- Hard vs soft byte budget limits