Rebrand SooSeF to FieldWitness

Complete project rebrand for better positioning in the press freedom
and digital security space. FieldWitness communicates both field
deployment and evidence testimony — appropriate for the target audience
of journalists, NGOs, and human rights organizations.

Rename mapping:
- soosef → fieldwitness (package, CLI, all imports)
- soosef.stegasoo → fieldwitness.stego
- soosef.verisoo → fieldwitness.attest
- ~/.soosef/ → ~/.fwmetadata/ (innocuous data dir name)
- SOOSEF_DATA_DIR → FIELDWITNESS_DATA_DIR
- SoosefConfig → FieldWitnessConfig
- SoosefError → FieldWitnessError

Also includes:
- License switch from MIT to GPL-3.0
- C2PA bridge module (Phase 0-2 MVP): cert.py, export.py, vendor_assertions.py
- README repositioned to lead with provenance/federation, stego backgrounded
- Threat model skeleton at docs/security/threat-model.md
- Planning docs: docs/planning/c2pa-integration.md, docs/planning/gtm-feasibility.md

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontends/web/templates/fieldkit/keys.html

@@ -1,8 +1,8 @@
 {% extends "base.html" %}
-{% block title %}Keys — SooSeF{% endblock %}
+{% block title %}Keys — FieldWitness{% endblock %}
 {% block content %}
 <h2><i class="bi bi-key me-2"></i>Key Management</h2>
-<p class="text-muted">Manage Stegasoo channel keys and Verisoo Ed25519 identity.</p>
+<p class="text-muted">Manage Stego channel keys and Attest Ed25519 identity.</p>
 
 <div class="row g-4">
     {# Channel Key #}
@@ -13,7 +13,7 @@
                 {% if keystore.has_channel_key %}
                 <p class="text-muted small">
                     Fingerprint: <code>{{ keystore.channel_fingerprint }}</code><br>
-                    Used for Stegasoo deployment isolation.
+                    Used for Stego deployment isolation.
                 </p>
                 {% else %}
                 <p class="text-muted small">No channel key configured.</p>
@@ -36,7 +36,7 @@
                 {% if keystore.has_identity %}
                 <p class="text-muted small">
                     Fingerprint: <code>{{ keystore.identity_fingerprint }}</code><br>
-                    Used for Verisoo attestation signing.
+                    Used for Attest attestation signing.
                 </p>
                 {% else %}
                 <p class="text-muted small">No identity configured.</p>