docs: architecture map (initial 5-slice generation, 105 pages)

Five-slice navigable map of the entire codebase under
docs/architecture-map/. Each slice is a self-contained
breadcrumbed sub-tree:

  01-frontend (16)              Svelte/SvelteKit UI
  02-tauri-runtime (26)         src-tauri commands + lifecycle
  03-audio-transcription (16)   audio + transcription crates
  04-llm-formatting-mcp (19)    llm, ai-formatting, mcp, cloud
  05-core-storage-hotkey-build  core, storage, hotkey, workspace,
                          (26) CI, dev glue

Plus master README.md and data-flow-end-to-end.md tracing
audio bytes from microphone to FTS5 search to MCP read.

Generated by 5 parallel subagents on 2026/05/09 against
HEAD 3c47000. Each page has YAML frontmatter, file:line code
refs, sibling cross-links, plain-English summaries.

Aggregated debt surfaced (full lists in master README):
RB-08 macOS power assertion, schema head drift v14 vs v15,
VAD blocked on ort version conflict, streaming primitives
not wired into live.rs, no prompt versioning, MCP has no
auth, cloud-providers in-memory keystore, SettingsPage
2 484 LOC, commands/live.rs 1 737 LOC, dual theme system,
brand rename to Lumenote pending across the codebase.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/architecture-map/03-audio-transcription/cargo-features.md

@@ -0,0 +1,70 @@
+---
+name: Cargo feature matrix
+type: architecture-map-page
+slice: 03-audio-transcription
+last_verified: 2026/05/09
+---
+
+# Cargo feature matrix
+
+> **Where you are:** [Architecture map](../README.md) → [Audio + Transcription](README.md) → Cargo features
+
+**Plain English summary.** `magnotia-audio` has no Cargo features. `magnotia-transcription` has two: `whisper` (default on, gates `whisper-rs`, `whisper_rs_backend.rs`, and `load_whisper`) and `whisper-vulkan` (default on, additive Vulkan GPU offload). Parakeet is unconditional. The defaults give a desktop GPU build; non-Vulkan or cloud-only configurations turn features off explicitly.
+
+## At a glance
+
+- Crate file: `crates/transcription/Cargo.toml`
+- Source of `[features]`: `crates/transcription/Cargo.toml:8-21`.
+- Source of conditionals in code: `#[cfg(feature = "whisper")]` in `crates/transcription/src/lib.rs:6` and `:10`, plus the gated `pub mod whisper_rs_backend;` and `load_whisper` re-export. Vulkan check inside `WhisperRsBackend::transcribe_sync` (`crates/transcription/src/whisper_rs_backend.rs:82`).
+
+## Feature table
+
+| Feature | Default | Pulls in | Gates |
+|---|---|---|---|
+| `whisper` | yes | `whisper-rs 0.16` (optional dep) | `whisper_rs_backend` module, `load_whisper` re-export, the `set_initial_prompt` path |
+| `whisper-vulkan` | yes | `whisper-rs?/vulkan` | Compile-time GPU offload. Runtime gated additionally on `vulkan_loader_available()` |
+
+Other deps are unconditional (always present): `transcribe-rs 0.3` (Parakeet), `magnotia-core`, `tokio`, `reqwest`, `futures-util`, `sha2`, `thiserror`, `tracing`. Dev-only: `tempfile`, `num_cpus` (used by `thread_sweep.rs` to label physical vs logical core counts).
+
+## Build commands
+
+Default (Whisper + Vulkan):
+
+```sh
+cargo build -p magnotia-transcription
+```
+
+Whisper without Vulkan (CPU-only desktop, Android without GPU drivers):
+
+```sh
+cargo build -p magnotia-transcription --no-default-features --features whisper
+```
+
+No Whisper at all (Parakeet only — useful for shrinking the binary and dropping `whisper-rs-sys`):
+
+```sh
+cargo build -p magnotia-transcription --no-default-features
+```
+
+The `--no-default-features` form drops the `whisper_rs_backend` module entirely, so any sibling code holding a `WhisperRsBackend` will fail to compile. `LocalEngine`, `SpeechModelAdapter`, `load_parakeet`, and the streaming primitives stay available.
+
+## Rationale (from Cargo.toml comments)
+
+- `whisper` exists as a feature so a future Windows non-AVX2 build, or a cloud-only ASR config, can drop `whisper-rs-sys` entirely (brief item #13). Disabling it also drops the `WhisperRsBackend` module and the `load_whisper` entry point.
+- `whisper-vulkan` is a separate feature so a non-Vulkan target (Android without GPU drivers, a CPU-only Windows build) can pull in `whisper-rs` but skip the Vulkan backend.
+- `transcribe-rs` is unconditional because today's only second engine (Parakeet) goes through it. Brief item #13's hypothetical "drop both engines" case would need a second feature flag introduced here.
+
+## Watch-outs
+
+- **`whisper-vulkan` is purely additive.** Disabling it does not change `whisper`'s default build except for skipping the Vulkan backend selection. There is no separate "force CPU" or "force GPU" runtime flag; the backend probes `vulkan_loader_available()` and uses Vulkan if present.
+- **`thiserror` and `tracing` are not feature-gated.** Comments justify keeping them unconditional (small cost, broad usage).
+- **Reqwest features are pinned.** `default-features = false, features = ["rustls-tls", "stream"]`. There is no `native-tls` fallback path; `--features reqwest/native-tls` would rebuild the dep with the wrong TLS backend on Linux.
+- **`whisper-rs` is `default-features = false, optional = true`.** Whisper-rs's own default features can re-enable backends we don't want; setting `default-features = false` keeps the dep minimal and lets `whisper-vulkan` add the one feature we care about.
+- **Switching feature sets between builds invalidates the build cache.** A common slow-down for first-time contributors switching `--no-default-features` builds.
+
+## See also
+
+- [Transcription whisper](transcription-whisper.md) — the gated module.
+- [Transcription parakeet](transcription-parakeet.md) — note Parakeet has no feature flag.
+- [Build tokenizers guard](build-tokenizers-guard.md) — Windows MSVC defence is part of the same brief item #6 thinking.
+- `crates/transcription/Cargo.toml` — the source of truth.