docs: reorganise documentation; derive header version from package.json; release v5.7.2

No protocol or message-protection changes.

The version in the application header was a literal and had fallen behind,
showing v5.6.0 while running 5.7.1. It now comes from package.json, and a test
fails if a hard-coded one reappears or if meta.json, the README badge, the
changelog and the docs disagree about the release.

Documentation reorganised so that everything technical lives in doc/ with an
index, and the root keeps only what belongs there by convention: README,
SECURITY, CHANGELOG and LICENSE.

- SECURITY.md rewritten. It listed a supported release line three major versions
  out of date and made claims the software does not make. It now states what is
  guaranteed, what is not, and how to report a problem.
- SECURITY_DISCLAIMER.md and RESPONSIBLE_USE.md merged into doc/USE-POLICY.md,
  which says what the software cannot protect against rather than listing
  generic advice.
- doc/SECURITY-ARCHITECTURE.md renamed to doc/ARCHITECTURE.md and rewritten
  around the session lifecycle, what verification gates, and how recovery works.
- doc/CRYPTOGRAPHY.md rewritten: key schedule, the Double Ratchet, framing, and
  memory handling, with values taken from the source rather than restated.
- doc/CONFIGURATION.md rewritten with the real file-type policy, ICE and TURN
  guidance, and the deployment caching rules that matter.
- docs/webrtc-config.md moved to doc/CALLS.md and rewritten; the obsolete
  docs/webrtc-audit.md, a working document full of stale line numbers, removed
  along with the docs/ directory.
- doc/CONTRIBUTING.md records what the recent regressions taught us about
  writing tests that can actually fail.
- doc/README.md added as an index.

Internal security review notes are excluded from the repository via .gitignore.
Those describe attack paths against specific releases in enough detail to
reproduce them, which is useful privately and harmful in public while users are
still updating.

doc/API.md

@@ -20,7 +20,7 @@
   connection. It is negotiated: both peers advertise `RATCHET_VERSION` in the
   offer and answer, and a peer that does not falls back to per-session keys.
 - `_ratchet.canEncrypt` is false on the joining peer until the inviting peer's
-  first message arrives — the sending chain does not exist until then. Callers
+  first message arrives, because the sending chain does not exist until then. Callers
   must check it rather than assume; the send path falls back to session keys for
   those first frames.
 - `_ratchet.getState()` returns counters and the number of retained keys for
@@ -52,7 +52,7 @@
 - The `isVoice` a callback receives is the **receiver's** verdict, not the
   sender's claim: `validateIncomingMetadata` clears it unless the transfer
   declares a recognised audio MIME type and fits the per-note and per-session
-  size budgets. A transfer that fails those checks is not rejected — it simply
+  size budgets. A transfer that fails those checks is not rejected; it simply
   loses the consent-free shortcut and is offered as a normal file.
 
 ## EnhancedSecureFileTransfer